There is no specific vtkUniformGrid XML writer so it should be
written out as a vtkImageData. It will miss the blanking information
since reading it back in it will be a vtkImageData. Also improved
the error output so that instead of just a type number, the class
name is also reported when vtkXMLDataSetWriter doesn't know
what type of writer to use.

Change-Id: I94b2961cd247f2bde782b36c87d345a0d106a7cd