lib/pull: Avoid journaling 404s for optional content

lib/pull: Avoid journaling 404s for optional content

Currently in Fedora we don't sign summaries, and every use of
`rpm-ostree` would emit to the journal an error when we failed
to fetch it.

Fix this by having `OSTREE_FETCHER_REQUEST_OPTIONAL_CONTENT` tell the fetcher
not to journal 404 errors. While fixing this, we had a mix of two booleans vs
the flags; fix things so we consistently use the flags in the fetcher and pull
code.

Closes: #1004
Approved by: mbarnes