Revert "Made incorrect username and password show the same error, for se...

...curity purposes."

This reverts commit 8d03cfc6b59b82900badbc9b4397f079e11ee2e1.

The security of obscuring usernames on the login form is useless when the
registration form, and the forgot password interfaces are not protected.
In addition Challenges feature reveals plenty of general usernames
and the Hall feature reveals all valuable usernames.

The usability disadvantage of not showing a proper error is real, however.