Mark synthetic as only available in vcl_error

Eventually, we want to be able to do synthetic everywhere, but for
now, mark it as just available in vcl_error.

Fixes: #936

bin/varnishtest/tests/r00936.vtc

+varnishtest "synthetic is only allowed in error"
+
+server s1 {
+	rxreq
+	txresp
+} -start
+
+varnish v1 -badvcl {
+	backend foo { .host = "127.0.0.1"; }
+
+	sub vcl_recv {
+		synthetic "HELLOO"; return (error);
+	}
+}
+

lib/libvcl/vcc_action.c

@@ -342,7 +342,7 @@ static struct action_table {
 	{ "return",		parse_return },
 	{ "rollback",		parse_rollback },
 	{ "set",		parse_set },
-	{ "synthetic",		parse_synthetic },
+	{ "synthetic",		parse_synthetic, VCL_MET_ERROR },
 	{ "unset",		parse_unset },
 	{ "purge",		parse_purge, VCL_MET_MISS | VCL_MET_HIT },
 	{ NULL,			NULL }