STYLE: Document and enhance ExtensionWizard.py

Add documentation of SlicerWizard.ExtensionWizard. Tweak create() and
addModule() commands to improve error reporting, and in particular for
addModule() to attempt to do nothing in case of an error (or at least,
to not write changes to the extension's CMakeLists.txt). Add a helper
wrapper around execute() to allow for much greater flexibility, e.g. if
it should cause the application to terminate (and newly do so
consistently) or return an exit code, and to allow CLI arguments to be
passed as method arguments (including gitpython-like kwargs support).

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22989 3bd1e089-480b-0410-8dfb-8563597acbee