function: Better message about call during GC

GJS can attempt to call back into JS during garbage collection if you
implement vfunc_remove() on your container class. (Destroying a container
will remove all its child widgets, and if you have implemented
vfunc_remove() in JS then that will cause a JS call.)

Note this possibility in the warning message, and also add whatever
information we can get about the offending callback without calling into
JSAPI.

https://bugzilla.gnome.org/show_bug.cgi?id=785040