Return specific error/code from create admin API call

[chancez]

It would be great to get an HTTP 409, and/or a better error when attempting to create a user that already exists. Currently I get the following type back from the generated client:

&googleapi.Error{Code:400, Message:"", Body:"{\"error\":\"bad_request\",\"error_description\":\"Email already in use.\"}", Errors:[]googleapi.ErrorItem(nil)}

Additionally, having a function I can use like IsErrorAlreadyExists(err error) bool would be the preferred way to check, rather than having to guess how to inspect the error type.

Similar to #470