If renaming a tag fails, we give a fairly generic error message ("failed to rename the tag"). This message is raised in the tags controller in rename, where we only know that it failed (but not why).

A common reason for a rename to fail is that the target tag exists. Can we check for that case and have a specific error message like "a tag with that name already exists"? This would mean that errors for other reasons (like malformed names) would still have a different (generic) message than the merge case.

The relevant code is in TagsController.rename.