CMakeLists: Tweak "unknown python version" error message text
CMakeLists: Tweak "unknown python version" error message text

thewtex/python-cmake-buildsystem,davidsansome/python-cmake-buildsystem,adrianbroher/python-cmake-buildsystem,python-cmake-buildsystem/python-cmake-buildsystem,thewtex/python-cmake-buildsystem