For the CallbackGuard, print something to stderr and use abort() instead of exit()
For the CallbackGuard, print something to stderr and use abort() instead of exit()

Otherwise the process just silently exits without anybody knowing what's
wrong. With abort() we also make sure that no at_exit handlers are
called and it's easier to attach a debugger and/or get a coredump.

Fixes https://github.com/gtk-rs/glib/issues/224

rust-gnome/glib,EPashkin/rust-gnome-glib