CheckEventLog: incorrect error handling for failed EvtQuery

This is noted from MSDN documentation for [EvtQuery](http://msdn.microsoft.com/en-us/library/windows/desktop/aa385466(v=vs.85).aspx):
> A handle to the query results if successful; otherwise, NULL. **If the function returns NULL, call the GetLastError function to get the error code.**

Currently, the error message is not useful. Here is a simple fix.