chromium/updater: Allow a single plugin arch

Until now, if we have a failure to fetch either the 32bit Debian package
or the 64bit Debian package, neither of these will be put into
sources.nix.

Unfortunately the beta/dev channels do not have a 32bit Debian package,
so even though there is a 64bit Debian package available we don't get
plugins *at* *all*.

This also introduces a nicer error message rather than just failing with
an assertion in fetchurl because we did not provide url/urls.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>