MINOR: http-htx: Use a dedicated function to check http reply validity

MINOR: http-htx: Use a dedicated function to check http reply validity

A dedicated function is added to check the validity of an http reply object,
after parsing. It is used to check the validity of http return rules.

For now, this function is only used to find the right error message in an
http-errors section for http replies of type HTTP_REPLY_ERRFILES (using
"errorfiles" argument). On success, such replies are updated to point on the
corresponding error message and their type is set to HTTP_REPLY_ERRMSG. If an
unknown http-errors section is referenced, anx error is returned. If a unknown
error message is referenced inside an existing http-errors section, a warning is
emitted and the proxy's error messages are used instead.