QmlDebug: Parse application output to track debugees state

Setting up the debugger connection might fail for a couple of reasons.
Check the application output to show the user a more detailed error
message.

Reviewed-by: hjk

src/plugins/debugger/debuggerengine.h

@@ -244,7 +244,7 @@ public:
     void handleCommand(int role, const QVariant &value);
 
     // Convenience
-    Q_SLOT void showMessage(const QString &msg, int channel = LogDebug,
+    Q_SLOT virtual void showMessage(const QString &msg, int channel = LogDebug,
         int timeout = -1) const;
     Q_SLOT void showStatusMessage(const QString &msg, int timeout = -1) const;