[http] Report unsuccessful response status lines at DBGVL_LOG

The precise HTTP response status code is currently visible only at
DBGLVL_EXTRA.  Allow for easier debugging by reporting the whole
status line at DBGLVL_LOG for any unsuccessful responses.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/net/tcp/httpcore.c

 		response_rc = -EIO_OTHER;
 	}
 	http->response.rc = response_rc;
+	if ( response_rc )
+		DBGC ( http, "HTTP %p status %s\n", http, status );
 
 	return 0;
 }