Fully define the API for sending replies from the `StationHandler:handle_rpc/2` callback

[wfvining]

The return value from StationHandler:handle_rpc/2 must allow a CSMS to perform the following actions:

1. send an OCPP response message
   • the response message should be represented as an Erlang term.
2. indicate an internal error
3. indicate an internal error and close the connection
4. close the connection without sending a response
5. defer sending a response