operations: Return structured errors on conflict in UserMergeBranch

While we already handle some of the errors in UserMergeBranch specially via structured errors, we don't yet do so for merge conflicts. Amend this error handling so that callers can know that the error is indeed caused by a merge conflict, and so that they can find our exactly which files have been conflicting.

Note that this is not done behind a feature flag: there aren't any callers which parse the error, and neither should there be any. So given that we retain the same error code as before but only amend the error message and add some error details this should not be a breaking change.

Closes #4066 (closed)