[udp] Propagate transmission errors to UDP interface users

Suggested-by: Simon Rowe <simon.rowe@eu.citrix.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/net/udp.c

 			      struct xfer_metadata *meta ) {
 
 	/* Transmit data, if possible */
-	udp_tx ( udp, iobuf, ( ( struct sockaddr_tcpip * ) meta->src ),
-		 ( ( struct sockaddr_tcpip * ) meta->dest ), meta->netdev );
-
-	return 0;
+	return udp_tx ( udp, iobuf, ( ( struct sockaddr_tcpip * ) meta->src ),
+			( ( struct sockaddr_tcpip * ) meta->dest ),
+			meta->netdev );
 }
 
 /** UDP data transfer interface operations */