Debugger: Warn about unsupported shell commands when using cdb

Task-number: QTCREATORBUG-6927
Change-Id: I20e2311d5069da9315a7b5a5bc8d698fe5e6e52b
Reviewed-by: hjk <hjk@qt.io>

src/plugins/debugger/cdb/cdbengine.cpp

@@ -456,6 +456,9 @@ void CdbEngine::setupEngine()
     if (debug)
         qDebug(">setupEngine");
 
+    if (!prepareCommand())
+        return;
+
     init();
     if (!m_logTime.elapsed())
         m_logTime.start();