feat: improve handshake error handling

When HelloReply's status is ERROR, throw an exception containing error
detail that can be handled by the caller.

gravitee-exchange-connector/gravitee-exchange-connector-websocket/src/main/java/io/gravitee/exchange/connector/websocket/channel/WebSocketConnectorChannel.java

@@ -16,7 +16,6 @@
 package io.gravitee.exchange.connector.websocket.channel;
 
 import io.gravitee.exchange.api.channel.exception.ChannelException;
-import io.gravitee.exchange.api.channel.exception.ChannelInitializationException;
 import io.gravitee.exchange.api.command.Command;
 import io.gravitee.exchange.api.command.CommandAdapter;
 import io.gravitee.exchange.api.command.CommandHandler;
@@ -75,7 +74,7 @@ public Completable initialize() {
                                 this.targetId = reply.getPayload().getTargetId();
                                 this.active = true;
                             } else {
-                                throw new ChannelInitializationException("Unable to parse hello reply payload");
+                                throw new WebSocketConnectorException(String.format("Hello handshake failed: %s", reply.getErrorDetails()), false);
                             }
                         });
                 })