GDBus: fix NotImplementedError message
GDBus: fix NotImplementedError message

Single quotes must be used, or the method_name concatenation is not
recognized.

https://bugzilla.gnome.org/show_bug.cgi?id=675018

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