Invitations: Clearer Message When A Link Has Already Been Used

Clicking an invitation link a second time — after it's already been accepted — now returns a message that says what to do: log in with your email and password, or use "Forgot password?" on the log-in page to reset it. Before, the response was a generic "token is not longer valid" error that gave no next step and, on the backend, was being reported as a system error even though re-clicking a used link is ordinary user behavior. The message and the response type are now correct for what actually happened.