Updating the metadata at the end of adding a remote means that it fails
if we're offline, even if we have the .flatpakrepo file available.
Unlike the other error cases in the process, which mean the operation
really has messed up, the remote is actually usable even if the
metadata update fails, because future operations will grab the
summary and any new metadata - so we should change this to a warning
to allow the operation to succeed when we're offline.
https://github.com/flatpak/flatpak/issues/917
f2dbaecf86