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
  it around.