Fix cmake check for minimum supported Python version

One of the VTK dashboards had problems evaluating PYTHON_VERSION,
so stick with PYTHON_MAJOR_VERSION and PYTHON_MINOR_VERSION instead.
Also, change SEND_ERROR to FATAL_ERROR.

Utilities/Python/CMakeLists.txt

@@ -7,15 +7,15 @@ include(vtkWrapPython)
 set(_message "Python ${PYTHON_MAJOR_VERSION}.${PYTHON_MINOR_VERSION} is too old, use Python 2.7 or 3.3+")
 set(_warning "Python ${PYTHON_MAJOR_VERSION}.${PYTHON_MINOR_VERSION} support is deprecated, use Python 2.7 or 3.3+")
 if(PYTHON_MAJOR_VERSION EQUAL 3)
-  if(PYTHON_VERSION VERSION_LESS 3.2)
-    message(SEND_ERROR ${_message})
-  elseif(NOT VTK_LEGACY_SILENT AND PYTHON_VERSION VERSION_LESS 3.3)
+  if(PYTHON_MINOR_VERSION LESS 2)
+    message(FATAL_ERROR ${_message})
+  elseif(NOT VTK_LEGACY_SILENT AND PYTHON_MINOR_VERSION LESS 3)
     message(WARNING ${_warning})
   endif()
 else()
-  if(PYTHON_VERSION VERSION_LESS 2.6)
-    message(SEND_ERROR ${_message})
-  elseif(NOT VTK_LEGACY_SILENT AND PYTHON_VERSION VERSION_LESS 2.7)
+  if(PYTHON_MINOR_VERSION LESS 6)
+    message(FATAL_ERROR ${_message})
+  elseif(NOT VTK_LEGACY_SILENT AND PYTHON_MINOR_VERSION LESS 7)
     message(WARNING ${_warning})
   endif()
 endif()