various: Improve error reporting if missing file server module on 0.12 If there is some error loading net.http.files then it would be swallowed by the pcall and then it would proceed to trying mod_http_files, which might cause unexpected behavior on 0.12 Ref #1765

This module lets you block connections to any remote servers not on a
whitelist.

``` {.lua}
s2s_whitelist = {
    "example.org",
}
```