MINOR: actions: Use ACT_RET_CONT code to ignore an error from a custom action

Some custom actions are just ignored and skipped when an error is encoutered. In
that case, we jump to the next rule. To do so, most of them use the return code
ACT_RET_ERR. Currently, for http rules and tcp content rules, it is not a
problem because this code is handled the same way than ACT_RET_CONT. But, it
means there is no way to handle the error as other actions. The custom actions
must handle the error and return ACT_RET_DONE. For instance, when http-request
rules are processed, an error when we try to replace a header value leads to a
bad request and an error 400 is returned to the client. But when we fail to
replace the URI, the error is silently ignored. This difference between the
custom actions and the others is an obstacle to write new custom actions.

So, in this first patch, ACT_RET_CONT is now returned from custom actions
instead of ACT_RET_ERR when an error is encoutered if it should be ignored. The
behavior remains the same but it is now possible to handle true errors using the
return code ACT_RET_ERR. Some actions will probably be reviewed to determine if
an error is fatal or not. Other patches will be pushed to trigger an error when
a custom action returns the ACT_RET_ERR code.

This patch is not tagged as a bug because it is just a design issue. But others
will depends on it. So be careful during backports, if so.
diff --git a/src/dns.c b/src/dns.c
index 64bb0d1..5ecb469 100644
--- a/src/dns.c
+++ b/src/dns.c
@@ -2247,7 +2247,7 @@
 
 	fqdn = smp->data.u.str.area;
 	if (action_prepare_for_resolution(s, fqdn) == -1)
-		return ACT_RET_ERR;
+		return ACT_RET_CONT; /* on error, ignore the action */
 
 	s->dns_ctx.parent = rule;
 	dns_link_resolution(s, OBJ_TYPE_STREAM, 0);