Users: User already exists error message

[jlbooker]

When attempting to add a new user from the Users module control panel (and not from the public-facing sign-up), if the username already exists, show an error message that specifically says the user already exists.

The current message of "invalid username" (or some such) confuses site administrators who may not realize the user already exists.

[Stardog]

Done.
8ebca8f