swc-windows-installer.py: Only create the python-scripts dir if it doesn't exist

swc-windows-installer.py: Only create the python-scripts dir if it doesn't exist

os.makedirs, unlike 'mkdir -p', errors if the target directory already
exists [1,2].

[1]: http://docs.python.org/2/library/os.html#os.makedirs
     "Raises an error exception if the leaf directory already exists
     or cannot be created"
[2]: http://docs.python.org/3/library/os.html#os.makedirs
     "If exist_ok is False (the default), an OSError is raised if the
     target directory already exists."