scanner: complete the enum-to-error-quark fix

scanner: complete the enum-to-error-quark fix

Turns out that the problem was not only in the wrong matching
to GType enums, but also that the non-GType heuristics used
to_underscores instead of to_underscores_noprefix, turning DBusError
into D_Bus_Error instead of DBus_Error.
Complete with various tests.

https://bugzilla.gnome.org/show_bug.cgi?id=669350