Merge request error display
Created by: dblessing
Fixes #8432
Currently, GitLab displays an error message whenever a new merge request is created. This is often confusing for users. There is another catch point that will show an error (see screenshot) if a user clicks "Compare branches" without specifying a source or target. If a source and target branch are not specified under other circumstances I think it's OK to quietly 'fail' without showing a message. I'm not sure if this is acceptable but I can't see any pitfalls so far. Please provide feedback.
Existing and unchanged (if user clicks 'Compare branches' without specifying'):