mod_http_errors: Make it easier to override 'http-message' handler

Per the unspoken internal standard of -1 as priority for most built-in
event handlers, since this makes it easy for 3rd party plugins to
override behavior by hooking at the default priority of 0.

plugins/mod_http_errors.lua

@@ -83,7 +83,7 @@ module:hook("http-message", function (event)
 		event.response.headers.content_type = "text/html; charset=utf-8";
 	end
 	return render(html, event);
-end);
+end, -1);
 
 local icon = [[
 <svg xmlns="http://www.w3.org/2000/svg" height="0.7em" viewBox="0 0 480 480" width="0.7em">