account-widget: use the "error" CSS class on wrong entries

That's the proper way to do as GTK+ defines this class for exactly this
purpose. Our current code was pretty hacky anyway.

This also has the nice side effect of fixing a bug making the text unreadable
when selecting it from the entry.

https://bugzilla.gnome.org/show_bug.cgi?id=710252
remove tpaw_make_color_whiter()

It's not used any more.

https://bugzilla.gnome.org/show_bug.cgi?id=710252