Storage/S3: Fix source backend check
Storage/S3: Fix source backend check

Also warn if non-fs backends are used with --rclone, as this only works
if they produce filesystem paths that are still valid after leaving the
context manager returning them.

ThiefMaster/indico-plugins,ThiefMaster/indico-plugins,indico/indico-plugins,ThiefMaster/indico-plugins,indico/indico-plugins,indico/indico-plugins,indico/indico-plugins,ThiefMaster/indico-plugins