BUG: error for non-fatal xml issue
As the original author indicated [1], this was meant to be a warning,
but the use of vtkErrorMacro indicates and error even when
the read is successful.
For example, reading an empty vtkPolyData results
in an error message even though the object is correctly read.
This leads to backwards incompatible behavior as discussed in [2], where the existence of error messages is interpreted as meaning that the read failed.
[1] 7f768465