Fix error handling in `PixbufLoader`
Fix error handling in `PixbufLoader`

No longer requires passing an `Error`, and returns `Result<(), Error> instead of
bool.

hmeyer/gdk,gkoz/rust-gnome-gdk,hmeyer/gdk,gkoz/gtk-rs-gdk,gkoz/gtk-rs-gdk,gkoz/rust-gnome-gdk,rust-gnome/gdk,rust-gnome/gdk