Make AlertMessage toString contain the actual alert message.

core/src/main/java/com/google/bitcoin/core/AlertMessage.java

@@ -57,6 +57,11 @@ public class AlertMessage extends Message {
         super(params, payloadBytes, 0);
     }
 
+    @Override
+    public String toString() {
+        return "ALERT: " + getStatusBar();
+    }
+
     @Override
     void parse() throws ProtocolException {
         // Alerts are formatted in two levels. The top level contains two byte arrays: a signature, and a serialized