Fix handling of save errors in file-uri in two ways:
- don't try to check if saving to a directly mounted file succeeded,
the mount (e.g. via FUSE) might allow writing, but not stat()ing.
Instead, rely on the return value of gimp_file_save().
- for non-mounted files, provide a separate error message for the case
where the temporary file contains nothing, which is IMO useless
paranoia, but the check was probably added for a reason, so keep