gmain: Warn when g_source_remove() fails

Trying to remove a non-existent source should really be
a programming error, as the programmer could be trying to
use the wrong function to remove a callback, as seen when
GtkScrolledWindow tried to remove ID from another function
using g_source_remove().

See https://bugzilla.gnome.org/show_bug.cgi?id=710666#c12

https://bugzilla.gnome.org/show_bug.cgi?id=710724
gmain: test g_source_remove() with invalid ID

Make sure we get the proper critical displayed.

https://bugzilla.gnome.org/show_bug.cgi?id=710724
tests: expect critical on failure to remove source

We've added a g_critical() on failure to remove sources, so make sure we
expect to see that (instead of failing the test due to the unexpected
message).

https://bugzilla.gnome.org/show_bug.cgi?id=710724