STYLE: More refactoring of SlicerWizard

Also move manipulation of extension CMakeLists.txt to its own Python
module, and adjust code style to prefer raising exceptions in modules
rather than calling exit(). (We still do call exit(), but it is moved to
higher up the stack where we now catch the newly raised exceptions.)

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22951 3bd1e089-480b-0410-8dfb-8563597acbee