Add getLastError() API to MultiProtocolJSONClient

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.

ocpp-v2/src/main/java/eu/chargetime/ocpp/MultiProtocolJSONClient.java

@@ -206,6 +206,10 @@ public void disconnect() {
     client.disconnect();
   }
 
+  public Exception getLastError() {
+    return transmitter.getLastError();
+  }
+
   @Override
   public boolean isClosed() {
     return transmitter.isClosed();

ocpp-v2/src/main/java/eu/chargetime/ocpp/MultiProtocolWebSocketTransmitter.java

@@ -54,6 +54,7 @@ public class MultiProtocolWebSocketTransmitter implements Transmitter {
   private final JSONConfiguration configuration;
   private final Draft draft;
 
+  private volatile Exception lastError = null;
   private volatile boolean closed = true;
   private volatile WebSocketClient client;
   private WssSocketBuilder wssSocketBuilder;
@@ -71,6 +72,7 @@ public MultiProtocolWebSocketTransmitter(
   public void connect(String uri, RadioEvents events) {
     final URI resource = URI.create(uri);
 
+    lastError = null;
     Map<String, String> httpHeaders = new HashMap<>();
     String username = configuration.getParameter(JSONConfiguration.USERNAME_PARAMETER);
     Object password = configuration.getParameter(JSONConfiguration.PASSWORD_PARAMETER);
@@ -120,6 +122,7 @@ public void onClose(int code, String reason, boolean remote) {
 
           @Override
           public void onError(Exception ex) {
+            lastError = ex;
             if (ex instanceof ConnectException) {
               logger.error("On error triggered caused by:", ex);
             } else {
@@ -261,6 +264,10 @@ public void send(Object request) throws NotConnectedException {
     }
   }
 
+  public Exception getLastError() {
+    return lastError;
+  }
+
   public boolean isClosed() {
     return closed;
   }