lib/repo-pull: Improve error message when no summary is found

lib/repo-pull: Improve error message when no summary is found

In ostree_repo_remote_fetch_summary_with_options(), if no summary is
found on the server and summary verification is enabled, the error
message implies that it's the summary signature that's missing, which is
misleading. This commit adds a more specific error message for the case
of a missing summary, which has the side effect of explicitly checking
for the case that signatures != NULL && summary == NULL after
repo_remote_fetch_summary(), even though that should never happen.

One effect of this is that if you run "flatpak remote-add" with an
incorrect URL you get a more helpful error message, and similarly for
other flatpak operations and other users of ostree.

Closes: #1522
Approved by: cgwalters