Throw an exception when registering a GType that already exists
Throw an exception when registering a GType that already exists

The GType system only logs a warning in that case, and it can
make bugs harder to track.

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

pixunil/cjs,endlessm/gjs,djdeath/gjs,pixunil/cjs,endlessm/gjs,endlessm/gjs,djdeath/gjs,mtwebster/cjs,djdeath/gjs,endlessm/gjs,djdeath/gjs,pixunil/cjs,mtwebster/cjs,mtwebster/cjs,pixunil/cjs,mtwebster/cjs,djdeath/gjs,mtwebster/cjs,pixunil/cjs,endlessm/gjs