mod_http: Soften dependency on mod_http_errors

This allows disabling mod_http_errors by adding it to
moduless_disabled and ensures mod_http loads even if the error pages
aren't as pretty.

plugins/mod_http.lua

@@ -7,7 +7,9 @@
 --
 
 module:set_global();
-module:depends("http_errors");
+pcall(function ()
+	module:depends("http_errors");
+end);
 
 local portmanager = require "core.portmanager";
 local moduleapi = require "core.moduleapi";