Show error message when adding an issue to epic fails

When I attempt to add a public issue to a confidential epic through the "epic tree widget" dialog, then no error message is displayed, only the text field is marked red:

The problem is probably because the EpicIssue link is not created (because validation fails) and ee/app/services/epic_issues/create_service.rb raises an exception (it calls link.save!) and this exception is not caught and no JSON response is returned in that case (only a generic 422 html reposnse is returned).

Update: the above screenshot is from staging, on production I get a generic "Issue not found error":

Reproducer steps:

• create a new confidential epic
• go the the epic page and select "Add an existing issue"
• select a not-confidential issue and click "Add"
• the issue is not added and not a clear message is displayed