It's sufficiently unique that it doesn't need the Elastic specifier. Actually, I wouldn't mind if we took "Elastic" off all of these and let the importers decide whether to qualify them. The only reason we didn't is legacy. Perhaps we'll deprecate the long names eventually.
Factor out _join_path to make testing easier.
Refactor code to determine what exception to raise and raise an `ElasticIndexAlreadyExistsError`, if appropriate.