Misleading error message when creating new branch

When creation of a new branch fails because we pass an invalid ref, then app/services/branches/create_service.rb always assumes that the branch creation failed because new branch name is invalid:

      new_branch = repository.add_branch(current_user, branch_name, ref)

      if new_branch
        success(new_branch)
      else
        error("Invalid reference name: #{branch_name}")
      end 

It would be nice to return correct error message if the branch creation fails because of invalid source reference.

A reproducer is the spec linked from the discussion bellow which uses GraphQL API.

The following discussion from !30388 (merged) should be addressed:

• @cwoolley-gitlab started a discussion: (+1 comment)

  This error message seems odd. I didn't dig into the code, but based on the test setup, I would expect the message to be Invalid reference name: unknown. That is, it should refer to the target branch or value of ref, not the source branch (which doesn't even exist yet, so it should be "invalid").