Show more appropriate error message when merging fails

Show a generic error when there are exceptions but still log
the error.

If no exception occurred but the commit wasn't created or other
exceptions was raised, show a generic failure message.

app/services/merge_requests/merge_service.rb

@@ -8,6 +8,8 @@ module MergeRequests
   # Executed when you do merge via GitLab UI
   #
   class MergeService < MergeRequests::MergeBaseService
+    GENERIC_ERROR_MESSAGE = 'An error occurred while merging'
+
     delegate :merge_jid, :state, to: :@merge_request
 
     def execute(merge_request, options = {})
@@ -79,7 +81,7 @@ module MergeRequests
       if commit_id
         log_info("Git merge finished on JID #{merge_jid} commit #{commit_id}")
       else
-        raise_error('Conflicts detected during merge')
+        raise_error(GENERIC_ERROR_MESSAGE)
       end
 
       merge_request.update!(merge_commit_sha: commit_id)
@@ -96,7 +98,7 @@ module MergeRequests
             "Something went wrong during merge pre-receive hook. #{e.message}".strip
     rescue => e
       handle_merge_error(log_message: e.message)
-      raise_error('Something went wrong during merge')
+      raise_error(GENERIC_ERROR_MESSAGE)
     end
 
     def after_merge

changelogs/unreleased/30141-merge-error-msg-update.yml

+---
+title: Show more appropriate error message when merging fails
+merge_request: 52671
+author:
+type: fixed

spec/features/merge_request/user_sees_merge_widget_spec.rb

@@ -388,7 +388,7 @@ RSpec.describe 'Merge request > User sees merge widget', :js do
       click_button 'Merge'
 
       page.within('.mr-widget-body') do
-        expect(page).to have_content('Conflicts detected during merge')
+        expect(page).to have_content('An error occurred while merging')
       end
     end
   end

spec/services/merge_requests/merge_service_spec.rb

@@ -310,12 +310,12 @@ RSpec.describe MergeRequests::MergeService do
       it 'logs and saves error if there is an exception' do
         error_message = 'error message'
 
-        allow(service).to receive(:repository).and_raise('error message')
+        allow(service).to receive(:repository).and_raise(error_message)
         allow(service).to receive(:execute_hooks)
 
         service.execute(merge_request)
 
-        expect(merge_request.merge_error).to include('Something went wrong during merge')
+        expect(merge_request.merge_error).to eq(described_class::GENERIC_ERROR_MESSAGE)
         expect(Gitlab::AppLogger).to have_received(:error).with(a_string_matching(error_message))
       end
 
@@ -343,9 +343,7 @@ RSpec.describe MergeRequests::MergeService do
         expect(Gitlab::AppLogger).to have_received(:error).with(a_string_matching(error_message))
       end
 
-      it 'logs and saves error if there is a merge conflict' do
-        error_message = 'Conflicts detected during merge'
-
+      it 'logs and saves error if commit is not created' do
         allow_any_instance_of(Repository).to receive(:merge).and_return(false)
         allow(service).to receive(:execute_hooks)
 
@@ -353,8 +351,8 @@ RSpec.describe MergeRequests::MergeService do
 
         expect(merge_request).to be_open
         expect(merge_request.merge_commit_sha).to be_nil
-        expect(merge_request.merge_error).to include(error_message)
-        expect(Gitlab::AppLogger).to have_received(:error).with(a_string_matching(error_message))
+        expect(merge_request.merge_error).to include(described_class::GENERIC_ERROR_MESSAGE)
+        expect(Gitlab::AppLogger).to have_received(:error).with(a_string_matching(described_class::GENERIC_ERROR_MESSAGE))
       end
 
       context 'when squashing is required' do