Invite User dialog: separation of errors

[hingobway]

Need to separate concept of USER_ALREADY_EXISTS from USER_ALREADY_REFERRED if not done so already and create a custom response to tell the user if that email corresponds with someone who has already been invited.

• separate error messages
• custom msg for user already existing
• custom msg for user that's already been invited
  • explain that user can't resend welcome email, but provide link they can send to them