MINOR: http-rules: Return an error when custom actions return ACT_RET_ERR

MINOR: http-rules: Return an error when custom actions return ACT_RET_ERR

Thanks to the commit "MINOR: actions: Use ACT_RET_CONT code to ignore an error
from a custom action", it is now possible to trigger an error from a custom
action in http rules. Now, when a custom action returns the ACT_RET_ERR code
from an http-request rule, an error 400 is returned. And from an http-response
rule, an error 502 is returned.

Be careful if this patch is backported. The other mentioned patch must be
backported first.