Be more clear that g_return_if_fail is undefined behaviour

In particular, it is not incorrect to g_return_if_fail (..., FALSE)
in a function returning a "success" gboolean and a GError: "failure to
meet the preconditions is an error" takes precedence over the
GError documentation's guarantee that the error will be set on failure.

Bug: https://bugzilla.gnome.org/show_bug.cgi?id=660809
Reviewed-by: Emmanuele Bassi