Improve error messages when adding a child epic and it fails

ee/app/models/ee/epic.rb

@@ -353,15 +353,15 @@ module EE
       preloaded_parent_group_and_descendants ||= parent.group.self_and_descendants
 
       if self == parent
-        errors.add :parent, 'Cannot add an epic as a child of itself'
+        errors.add :parent, "This epic cannot be added. An epic cannot be added to itself."
       elsif parent.children.to_a.include?(self)
-        errors.add :parent, "This epic can't be added as it is already assigned to the parent"
+        errors.add :parent, "This epic cannot be added. It is already assigned to the parent epic."
       elsif parent.has_ancestor?(self)
-        errors.add :parent, "This epic can't be added as it is already assigned to this epic's ancestor"
+        errors.add :parent, "This epic cannot be added. It is already an ancestor of the parent epic."
       elsif !preloaded_parent_group_and_descendants.include?(group)
-        errors.add :parent, "This epic can't be added because it must belong to the same group as the parent, or subgroup of the parent epic’s group"
+        errors.add :parent, "This epic cannot be added. An epic must belong to the same group or subgroup as its parent epic."
       elsif level_depth_exceeded?(parent)
-        errors.add :parent, "This epic can't be added as the maximum depth of nested epics would be exceeded"
+        errors.add :parent, "This epic cannot be added. One or more epics would exceed the maximum depth (#{MAX_HIERARCHY_DEPTH}) from its most distant ancestor."
       end
     end
 

ee/app/services/epic_links/create_service.rb

@@ -8,7 +8,7 @@ module EpicLinks
       end
 
       if issuable.max_hierarchy_depth_achieved?
-        return error("This epic can't be added because the parent is already at the maximum depth from its most distant ancestor", 409)
+        return error("This epic cannot be added. One or more epics would exceed the maximum depth (#{Epic::MAX_HIERARCHY_DEPTH}) from its most distant ancestor.", 409)
       end
 
       if referenced_issuables.count == 1

ee/spec/features/epics/epic_issues_spec.rb

@@ -29,6 +29,7 @@ RSpec.describe 'Epic Issues', :js do
 
   before do
     stub_licensed_features(epics: true, subepics: true)
+    stub_const('Epic', ::Epic, transfer_nested_constants: true)
   end
 
   def visit_epic
@@ -214,7 +215,7 @@ RSpec.describe 'Epic Issues', :js do
             add_epics(references)
 
             expect(page).to have_selector('.gl-field-error')
-            expect(find('.gl-field-error')).to have_text("This epic can't be added because the parent is already at the maximum depth from its most distant ancestor")
+            expect(find('.gl-field-error')).to have_text("This epic cannot be added. One or more epics would exceed the maximum depth (5) from its most distant ancestor.")
           end
         end
       end

ee/spec/services/epic_links/create_service_spec.rb

@@ -13,6 +13,10 @@ RSpec.describe EpicLinks::CreateService do
 
     let(:valid_reference) { epic_to_add.to_reference(full: true) }
 
+    before do
+      stub_const('Epic', ::Epic, transfer_nested_constants: true)
+    end
+
     shared_examples 'system notes created' do
       it 'creates system notes' do
         expect { subject }.to change { Note.system.count }.from(0).to(2)
@@ -85,7 +89,7 @@ RSpec.describe EpicLinks::CreateService do
 
             context 'when an epic from another group is given' do
               let(:other_group) { create(:group) }
-              let(:expected_error) { "This epic can't be added because it must belong to the same group as the parent, or subgroup of the parent epic’s group" }
+              let(:expected_error) { "This epic cannot be added. An epic must belong to the same group or subgroup as its parent epic." }
               let(:expected_code) { 409 }
 
               before do
@@ -97,7 +101,7 @@ RSpec.describe EpicLinks::CreateService do
 
             context 'when hierarchy is cyclic' do
               context 'when given child epic is the same as given parent' do
-                let(:expected_error) { 'Cannot add an epic as a child of itself' }
+                let(:expected_error) { "This epic cannot be added. An epic cannot be added to itself." }
                 let(:expected_code) { 409 }
 
                 subject { add_epic([epic.to_reference(full: true)]) }
@@ -106,7 +110,7 @@ RSpec.describe EpicLinks::CreateService do
               end
 
               context 'when given child epic is parent of the given parent' do
-                let(:expected_error) { "This epic can't be added as it is already assigned to this epic's ancestor" }
+                let(:expected_error) { "This epic cannot be added. It is already an ancestor of the parent epic." }
                 let(:expected_code) { 409 }
 
                 before do
@@ -117,7 +121,7 @@ RSpec.describe EpicLinks::CreateService do
               end
 
               context 'when new child epic is an ancestor of the given parent' do
-                let(:expected_error) { "This epic can't be added as it is already assigned to this epic's ancestor" }
+                let(:expected_error) { "This epic cannot be added. It is already an ancestor of the parent epic." }
                 let(:expected_code) { 409 }
 
                 before do
@@ -136,7 +140,7 @@ RSpec.describe EpicLinks::CreateService do
                 epic_to_add.update(parent: epic)
               end
 
-              let(:expected_error) { "This epic can't be added as it is already assigned to the parent" }
+              let(:expected_error) { "This epic cannot be added. It is already assigned to the parent epic." }
               let(:expected_code) { 409 }
 
               include_examples 'returns an error'
@@ -152,14 +156,14 @@ RSpec.describe EpicLinks::CreateService do
                 epic.update(parent: epic4)
               end
 
-              let(:expected_error) { "This epic can't be added because the parent is already at the maximum depth from its most distant ancestor" }
+              let(:expected_error) { "This epic cannot be added. One or more epics would exceed the maximum depth (5) from its most distant ancestor." }
               let(:expected_code) { 409 }
 
               include_examples 'returns an error'
             end
 
             context 'when total depth after adding would exceed depth limit' do
-              let(:expected_error) { "This epic can't be added as the maximum depth of nested epics would be exceeded" }
+              let(:expected_error) { "This epic cannot be added. One or more epics would exceed the maximum depth (5) from its most distant ancestor." }
               let(:expected_code) { 409 }
 
               before do

ee/spec/services/epics/tree_reorder_service_spec.rb

@@ -237,7 +237,7 @@ RSpec.describe Epics::TreeReorderService do
               epic2.update(parent: epic1)
             end
 
-            it_behaves_like 'error for the tree update', "This epic can't be added because it must belong to the same group as the parent, or subgroup of the parent epic’s group"
+            it_behaves_like 'error for the tree update', "This epic cannot be added. An epic must belong to the same group or subgroup as its parent epic."
           end
 
           context 'when user does not have permissions to admin the new parent' do