Show appropriate error message from Gitaly when merging fails

Summary

In #30141 (closed), we started showing a generic error message instead of showing "Conflicts detected during merge" because it is not always true.

Proposal

Handle errors from Gitaly as best as we can for the user. Don't just return the error message as it can contain information we don't want to expose publicly.

In a case of merge failure due to conflict, Gitaly should be able to tell clearly that it is indeed a conflict and it should be translated as a specific exception in gitlab-rails. This way, we can show a more user friendly message on why it fails.

Technical details

For now see: #321358 (comment 525777015).