mod_admin_socket: Return error on unhandled input to prevent apparent freeze
When mod_admin_socket is loaded without mod_admin_shell, attempt to use
`prosodyctl shell` will appear to freeze after any input, since no
response is returned.
For full information on our dependencies, version requirements, and
where to find them, see https://prosody.im/doc/depends
If you have luarocks available on your platform, install the following:
- luaexpat
- luasocket
- luafilesystem
- luasec