rgw: http client: print curl error messages during curl failures

rgw: http client: print curl error messages during curl failures

When curl fails, curl has a curl error code which can be useful to understand
the failure. Print this error message, as http status in these cases can likely
be 0, which makes it harder to debug. Example error message::

ERROR: msg->data.result=27 req_data->id=1 http_status=0
ERROR: curl error: Out of memory

Signed-off-by: Abhishek Lekshmanan <abhishek@suse.com>