Add error handling in prepare task (PROBLEM-1070)

src/CounterBasedOperation.cpp

@@ -259,7 +259,7 @@ throw (ni660Xsl::DAQException)
     }
 
     int err = DAQmxTaskControl(this->task_handle_, DAQmx_Val_Task_Verify);
-    if(err<0)
+    if(err<0 || err>0)
     {
 	this->state_ = UNKNOWN;
 	throw ni660Xsl::DAQException("user configuration invalid",