MEDIUM: http_error_message: txn->status / http_get_status_idx.

This commit removes second argument(msgnum) from http_error_message and
changes http_error_message to use s->txn->status/http_get_status_idx for
mapping status code from 200..504 to HTTP_ERR_200..HTTP_ERR_504(enum).

This is needed for http-request tarpit deny_status commit.
diff --git a/include/proto/proto_http.h b/include/proto/proto_http.h
index 6c81766..9409df3 100644
--- a/include/proto/proto_http.h
+++ b/include/proto/proto_http.h
@@ -136,7 +136,7 @@
 void free_http_req_rules(struct list *r);
 void free_http_res_rules(struct list *r);
 void http_reply_and_close(struct stream *s, short status, struct chunk *msg);
-struct chunk *http_error_message(struct stream *s, int msgnum);
+struct chunk *http_error_message(struct stream *s);
 struct redirect_rule *http_parse_redirect_rule(const char *file, int linenum, struct proxy *curproxy,
                                                const char **args, char **errmsg, int use_fmt, int dir);
 int smp_fetch_cookie(const struct arg *args, struct sample *smp, const char *kw, void *private);