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/src/filters.c b/src/filters.c
index 9ec794a..cafc449 100644
--- a/src/filters.c
+++ b/src/filters.c
@@ -1069,7 +1069,7 @@
 			http_reply_and_close(s, s->txn->status, NULL);
 		else {
 			s->txn->status = 400;
-			http_reply_and_close(s, 400, http_error_message(s, HTTP_ERR_400));
+			http_reply_and_close(s, 400, http_error_message(s));
 		}
 	}