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].


 
[1] vtk/vtk@7f768465

[2] https://github.com/Slicer/Slicer/issues/6447

IO/XML/vtkXMLUnstructuredDataReader.cxx

@@ -413,7 +413,7 @@ void vtkXMLUnstructuredDataReader::SetupOutputData()
   }
   else
   {
-    vtkErrorMacro(
+    vtkWarningMacro(
       "No Points element available in first piece found in file. Reading file may fail.");
   }