Improve error handling in ocpp/charge_point.py

[Jared-Newell-Mobility]

see #426

Table 7 of the Open Charge Point Protocol JSON 1.6, OCPP-J 1.6 Specification lists all error codes with a short description.

NotSupported - Requested Action is not known by receiver
NotImplemented - Requested Action is recognized but not supported by the receiver charge_point.py:175 can indicate both problems.
Trying to send a request with action "InvalidAction" triggers this code path. In this case, a NotSupportedError would be fitting.

However, the code path is also executed when the receiver receives a request for an action the receiver doesn't implement. For example, if a charger receives a "GetDiagnostics" request without supporting this feature. Now NotImplementedError would be suitable.

A while ago, we changed the error type from "NotImplementedErrortoNotSupportedError`. See c6e7e48

I'm hesitant to revert that change by approving this PR. Instead, some code should be include to distinguish between 2 scenarios I described and raise the appropriate error.