Report the zlib message if we have gunzip trouble

bin/varnishtest/vtc_http.c

@@ -546,7 +546,7 @@ cmd_http_gunzip_body(CMD_ARGS)
 	assert(Z_OK == inflateInit2(&vz, 31));
 	i = inflate(&vz, Z_FINISH);
 	if (i != Z_STREAM_END)
-		vtc_log(hp->vl, 0, "Gunzip error = %d", i);
+		vtc_log(hp->vl, 0, "Gunzip error = %d (%s)", i, vz.msg);
 	hp->bodyl = vz.total_out;
 	memcpy(hp->body, p, hp->bodyl);
 	free(p);