Improved exception messages in the XMLVTK reader

      // load and verify XML

      tinyxml2::XMLError status = dom.LoadFile( fileName.c_str() );

      if( status != XML_SUCCESS )

         throw MeshReaderError( "XMLVTK", "failed to parse the file as an XML document." );

         throw MeshReaderError( "XMLVTK", "failed to parse the file " + fileName + " as an XML document." );

      // verify root element

      const XMLElement* elem = dom.FirstChildElement();

      verifyElement( elem, "VTKFile" );

      if( elem->NextSibling() )

         throw MeshReaderError( "XMLVTK", "<VTKFile> is not the only element in the file" );

         throw MeshReaderError( "XMLVTK", "<VTKFile> is not the only element in the file " + fileName );

      // verify byte order

      const std::string systemByteOrder = (isLittleEndian()) ? "LittleEndian" : "BigEndian";