Surface specific Gitaly conflict reasons in the merge widget

What does this MR do and why?

A merge that failed on a git conflict showed only "An error occurred while merging. Try again.", and a failed squash showed a fixed "Squashing failed" string. In both cases Gitaly reports the real reason and we discarded it.

Gitaly returns a structured merge_conflict error on UserMergeBranch, but user_merge_branch had no branch for it, so the failure became a plain CommandError and MergeService replaced the message with a generic constant. user_squash handled resolve_revision and rebase_conflict in one branch, and SquashService rescued StandardError and dropped the message. Add Gitlab::Git::MergeConflictError, raise it from both RPCs, and map it to a specific message in each service. The raw Gitaly text is still logged for support.

References

#616263

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