all: clean-up usage of GError

all: clean-up usage of GError

Functions that take a GError** MUST fill it in on error. There is no
need to check whether error is NULL if the function it was passed to
had a failing return value.

Likewise, a proper GError must have a non-NULL message, so there's no
need to double-check that either.

Based-on-patch-by: Dan Winship <danw@gnome.org>