gnunet-download return code always 0

It seems that as long as gnunet-download get an output file and a URI it
always returns a zero (OK) result code.  I would expect it to return a
non-zero code if the download did not complete satisfactorily.  Instead,
it appears that I have to parse the debug output and/or check the
resulting file size to determine if an error occurred.

--kyler