backend.py prevent stack trace when not given overlay

[jfindlay]

Prevent a python stack trace in backend.py that can happen when invoking gs-pypi without arguments, for example.

Before

# gs-pypi
Traceback (most recent call last):
  File "/usr/lib/python-exec/python3.4/g-sorcery", line 18, in <module>
    sys.exit(g_sorcery.main())
  File "/usr/lib/python3.4/site-packages/g_sorcery/g_sorcery.py", line 68, in main
    return backend.instance(sys.argv[2:], config, global_config)
  File "/usr/lib/python3.4/site-packages/g_sorcery/backend.py", line 620, in __call__
    info_f = FileJSON(os.path.join(args.overlay, self.sorcery_dir),
  File "/usr/lib/python3.4/posixpath.py", line 82, in join
    path += b
TypeError: unsupported operand type(s) for +=: 'NoneType' and 'str'

After

# gs-pypi
usage: g-sorcery [-h] [-o OVERLAY] [-r REPOSITORY]
                 {sync,list,generate,generate-tree,install} ...
g-sorcery: error: No overlay specified