Raise custom exception in all methods of MergeService that want to inform the user
https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/1318#note_24255563
Instead of calling MergeService#log_merge_error(_, save_message_on_model: true)
, the methods on MergeService
that aren't #execute
should raise a custom MergeError
exception, so that we can handle it in one place, and the control flow is clearer.
This will need a custom EE MR as there are some methods that only exist in EE.