Revert WARNING to FATAL_ERROR in vtkWrapPython.
Revert WARNING to FATAL_ERROR in vtkWrapPython.

This reverts the part of commit b4a7e2b7 that turned the "missing header"
error into a warning.  If vtkWrapPython.cmake is unable to find a header
that it was told to wrap, that should qualify as an error.

keithroe/vtkoptix,sumedhasingla/VTK,keithroe/vtkoptix,SimVascular/VTK,SimVascular/VTK,sumedhasingla/VTK,keithroe/vtkoptix,keithroe/vtkoptix,sumedhasingla/VTK,SimVascular/VTK,SimVascular/VTK,SimVascular/VTK,SimVascular/VTK,keithroe/vtkoptix,sumedhasingla/VTK,keithroe/vtkoptix,keithroe/vtkoptix,sumedhasingla/VTK,SimVascular/VTK,sumedhasingla/VTK,SimVascular/VTK,sumedhasingla/VTK,keithroe/vtkoptix,sumedhasingla/VTK