[vtkusers] error codes from xml readers

[clinton at elemtech.com]

I'm using a vtkXMLUnstructuredGridReader and if I give it a file that doesn't 
exist, it gives me no error code when I try calling 
vtkAlgorithm::GetErrorCode().

When looking at the code, it seems some of the readers and writer set error 
codes and some don't.  Should they all be doing that?  I don't want to try 
parsing the output text messages to get the information I need.

Or is there some other way I get details about errors?

Thanks,
Clint