Better handle empty tokens for non-authed sessions or timeouts

[alanwill]

Here's an example traceback:

Traceback (most recent call last):
  File "./aws-credy.py", line 149, in <module>
    main()
  File "/Users/myname/Library/Python/3.8/lib/python/site-packages/click/core.py", line 829, in __call__
    return self.main(*args, **kwargs)
  File "/Users/myname/Library/Python/3.8/lib/python/site-packages/click/core.py", line 782, in main
    rv = self.invoke(ctx)
  File "/Users/myname/Library/Python/3.8/lib/python/site-packages/click/core.py", line 1066, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/Users/myname/Library/Python/3.8/lib/python/site-packages/click/core.py", line 610, in invoke
    return callback(*args, **kwargs)
  File "./aws-credy.py", line 26, in main
    profile_access_key, profile_secret_key, profile_token = get_role_credentials(
TypeError: cannot unpack non-iterable NoneType object