object: Don't crash on error in gjs_object_from_g_object()

Fix one spot where we ignore a JS exception from
gjs_lookup_object_prototype() which causes a crash later. Also fix
another spot where we would crash if JS_NewObjectWithGivenProto() fails,
though that is unlikely except on OOM.

https://bugzilla.gnome.org/show_bug.cgi?id=779918