Updated INVALID_USERNAME error to have a more accurate message.

Change-Id: Ifa3e18ba7479576109ac8e7d318ef42236d7eafc
Reviewed-on: http://review.couchbase.org/29044
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>