Store only generic message if rebase fails

Instead of storing detailed rebase error, only a generic message is
stored with MR. The reason is that this message is exposed and displayed
to end user and there is no reason to expose detailed backend
information. Error message is still logged so detailed information
can be found in logfile by admin if needed.

Related #41820

app/services/merge_requests/rebase_service.rb

 module MergeRequests
   class RebaseService < MergeRequests::WorkingCopyBaseService
+    REBASE_ERROR = 'Rebase failed. Please rebase locally'.freeze
+
     def execute(merge_request)
       @merge_request = merge_request
       if rebase
         success
       else
-        error('Failed to rebase. Should be done manually')
+        error(REBASE_ERROR)
       end
     end
@@ -22,8 +24,8 @@ module MergeRequests
       true
     rescue => e
-      log_error('Failed to rebase branch:')
-      log_error(e.message, save_message_on_model: true)
+      log_error(REBASE_ERROR, save_message_on_model: true)
+      log_error(e.message)
       false
     end
   end

changelogs/unreleased/4020-rebase-message.yml

+---
+title: Display user friendly error message if rebase fails.
+merge_request:
+author:
+type: fixed

spec/services/merge_requests/rebase_service_spec.rb

@@ -32,7 +32,7 @@ describe MergeRequests::RebaseService do
       it 'returns an error' do
         expect(service.execute(merge_request)).to match(status: :error,
-                                                        message: 'Failed to rebase. Should be done manually')
+                                                        message: described_class::REBASE_ERROR)
       end
     end
@@ -41,15 +41,15 @@ describe MergeRequests::RebaseService do
         allow(repository).to receive(:run_git!).and_raise('Something went wrong')
       end
-      it 'saves the error message' do
+      it 'saves a generic error message' do
         subject.execute(merge_request)
-        expect(merge_request.reload.merge_error).to eq 'Something went wrong'
+        expect(merge_request.reload.merge_error).to eq described_class::REBASE_ERROR
       end
       it 'returns an error' do
         expect(service.execute(merge_request)).to match(status: :error,
-                                                        message: 'Failed to rebase. Should be done manually')
+                                                        message: described_class::REBASE_ERROR)
       end
     end
@@ -58,15 +58,15 @@ describe MergeRequests::RebaseService do
         allow(repository).to receive(:run_git!).and_raise(Gitlab::Git::Repository::GitError, 'Something went wrong')
       end
-      it 'saves the error message' do
+      it 'saves a generic error message' do
         subject.execute(merge_request)
-        expect(merge_request.reload.merge_error).to eq 'Something went wrong'
+        expect(merge_request.reload.merge_error).to eq described_class::REBASE_ERROR
       end
       it 'returns an error' do
         expect(service.execute(merge_request)).to match(status: :error,
-                                                        message: 'Failed to rebase. Should be done manually')
+                                                        message: described_class::REBASE_ERROR)
       end
     end