Discussion on answer by blankip: Banned user tries to log in − show a “Banned” message or a generic “Cannot log you in”?

Imported from a comment discussion on http://ux.stackexchange.com/questions/86634/banned-user-tries-to-log-in-%e2%88%92-show-a-banned-message-or-a-generic-cannot-log-y/86718#86718