Expand branch creation error message

Currently when `repository#add_branch` returns `false` in
`Branches::CreateService` we return error message that the source
reference was invalid. This is the most likely cause of this error, but
not the only one.

Another cause is creating a branch with a subcomponent that matches the
full name of an existing branch, e.g. `feature/widget` and `feature`. In
this scenario it is the new branch name that is the problem, not the
source ref, but our error message gives no clues to this.

This is a particular problem for pull mirroring, where the SHA1 of the
source branch is used rather than a human-readable name. This requires
the user to copy down the SHA and go to the source repo to attempt to
understand which branch is causing the failure.

Ideally we would have separate errors for invalid source refs and
invalid branch names, but `repository#add_branch` hides the information
we need to distinguish between them.

This commit expands the error message to mention which branch we failed
to create, making it easier for users to find out they were creating
invalid branch names.

app/services/branches/create_service.rb

@@ -18,7 +18,7 @@ module Branches
       if new_branch
         success(new_branch)
       else
-        error("Invalid reference name: #{ref}")
+        error("Failed to create branch '#{branch_name}': invalid reference name '#{ref}'")
       end
     rescue Gitlab::Git::PreReceiveError => e
       Gitlab::ErrorTracking.track_exception(e, pre_receive_message: e.raw_message, branch_name: branch_name, ref: ref)

spec/requests/api/branches_spec.rb

@@ -719,10 +719,11 @@ RSpec.describe API::Branches do
     end
 
     it 'returns 400 if ref name is invalid' do
+      error_message = 'Failed to create branch \'new_design3\': invalid reference name \'foo\''
       post api(route, user), params: { branch: 'new_design3', ref: 'foo' }
 
       expect(response).to have_gitlab_http_status(:bad_request)
-      expect(json_response['message']).to eq('Invalid reference name: foo')
+      expect(json_response['message']).to eq(error_message)
     end
   end
 

spec/requests/api/graphql/mutations/branches/create_spec.rb

@@ -35,11 +35,12 @@ RSpec.describe 'Creation of a new branch' do
     end
 
     context 'when ref is not correct' do
+      err_msg = 'Failed to create branch \'another_branch\': invalid reference name \'unknown\''
       let(:new_branch) { 'another_branch' }
       let(:ref) { 'unknown' }
 
       it_behaves_like 'a mutation that returns errors in the response',
-                      errors: ['Invalid reference name: unknown']
+                      errors: [err_msg]
     end
   end
 end

spec/services/branches/create_service_spec.rb

@@ -38,11 +38,11 @@ RSpec.describe Branches::CreateService do
       end
 
       it 'returns an error with a reference name' do
-        error_message = 'Failed to create branch \'new-feature\': invalid reference name unknown'
+        err_msg = 'Failed to create branch \'new-feature\': invalid reference name \'unknown\''
         result = service.execute('new-feature', 'unknown')
 
         expect(result[:status]).to eq(:error)
-        expect(result[:message]).to eq('Invalid reference name: unknown')
+        expect(result[:message]).to eq(err_msg)
       end
     end
 

spec/services/commits/commit_patch_service_spec.rb

@@ -87,7 +87,7 @@ RSpec.describe Commits::CommitPatchService do
     context 'when specifying a non existent start branch' do
       let(:start_branch) { 'does-not-exist' }
 
-      it_behaves_like 'an error response', 'Invalid reference name'
+      it_behaves_like 'an error response', 'Failed to create branch'
     end
   end
 end