When there is a well-documented GError domain/code, use these,
especially since messages can be localized (not sure if that is the case
for this one, but theoretically it could) and change over time.

Also remove the "nice-to-have" comment on file-pdf-load since we won't
be changing it now, but leave it on file-pdf-load2 parameters which can
still change before 2.10.