Now that the search command is attempting to update appstream data for
all supported architectures, error messages get printed when it's
missing, which is often the case for i386. This commit changes
flatpak_dir_check_for_appstream_update() to only print an error if the
the appstream data for the host architecture is missing, but not if it's
a secondary architecture.
Closes: #1430
Approved by: alexlarsson
1e479095b2