twitter: bugfix for invalid OAuth PIN Entry

twitter: bugfix for invalid OAuth PIN Entry

There was a bug in "twitter pin" functionality where it didn't
check to verify that the PIN entered by the user was valid. As
a result, if the user entered an invalid PIN, the bot would
not respond as to whether or not twitter account binding was
successful. I replaced it with an error message if the PIN
is invalid. I also changed the error message for someone
who tries to enter a PIN without first using "twitter
authorize" to be more clear.