Change debug bad login echo to a more customisable badlogin.php page

I found this useful - I've submitted the pull request because you might as well. I hope you don't mind. I won't take offence if you reject it.

Test scenario: 
In original master. Enter bad username/password. See echo message
In forked master: Enter bad username/password. See OwnCloud login screen with bad credentials message above the sign in form. You can change what happens when a bad login occurs by customising the (new) badlogin.php file.