Added more descriptive error message to WPComLoginViewController.

Users who would login through this screen(when prompted by say the
Reader) wouldn't see the error message regarding 2FA that the user
should generate a app specific password because we had a hard coded
error message.
Merge pull request #1336 from wordpress-mobile/issue/1314-wrong-error-message-2fa

Added more descriptive error message to WPComLoginViewController.