exceptions: Add `ElasticIndexAlreadyExistsError` exception
client: Refactor exception raising code

Refactor code to determine what exception to raise and raise
an `ElasticIndexAlreadyExistsError`, if appropriate.
docs: Update index creation example
AUTHORS: Add me!
Take "Elastic" off the front of the IndexAlreadyExists exception.

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.
Tweak naming, style, etc.
Merge in the new IndexAlreadyExistsError. Closes #82.