Before this change, rclone serve sftp operating with a new rclone
after the md5sum/sha1sum detection was reworked to just run a plain
`md5sum`/`sha1sum` command in
3ea82032e7 sftp: support md5/sha1 with rsync.net #3254
Failed to signal to the remote that md5sum/sha1sum wasn't supported as
in
71e172a139 serve/sftp: support empty "md5sum" and "sha1sum" commands
We unconditionally return good hashes even if the remote being served
doesn't support the hash type in question.
This fix checks the hash type is supported and returns an error
MD5 hash not supported
When the backend is first contacted this will cause the sftp backend
to detect that the hash type isn't available.
Unfortunately this may have cached the wrong state so editing or
remaking the config may be necessary to fix it.
98fd00a655