CLEANUP: http-htx: Rename http_error structure into http_error_msg

The structure owns an error message, most of time loaded from a file, and
converted to HTX. It is created when an errorfile or errorloc directive is
parsed. It is renamed to avoid ambiguities with http_reply structure.
diff --git a/include/types/http_htx.h b/include/types/http_htx.h
index 95b64cc..b24b47f 100644
--- a/include/types/http_htx.h
+++ b/include/types/http_htx.h
@@ -71,7 +71,7 @@
 /* A custom HTTP error message load from a row file and converted in HTX. The
  * node key is the file path.
  */
-struct http_error {
+struct http_error_msg {
 	struct buffer msg;
 	struct ebpt_node node;
 };