Add getLastError() API to MultiProtocolJSONClient

[robert-s-ubi]

To allow determining the cause of a disconnection, add a new API to MultiProtocolJSONClient to retrieve the Exception passed from the WebSocket library in the onError() callback.

This is required to allow logging the OCPP 2.0.1 Security Events "InvalidTLSVersion" and "InvalidTLSCipherSuite", by examining the type of Exception and its message string.