Improve error when release branch already exists

If the release branch already exists, the action will fail with an error message that doesn't make it obvious why it failed. We should plainly state that the release branch already exists, and that it should be deleted to run the action.

No file or test is named in the issue. Locate the action entry point and the error path used when the release branch already exists, then inspect any existing tests for that failure. Done means the error clearly says the branch exists and should be deleted before rerunning the action.

Written by the indexing model from the issue text.