Nicer error when cwd is not playbook dir

[msabramo]

Instead of:

$ arm freeze
Traceback (most recent call last):
  File "/Users/marca/python/virtualenvs/smstack/bin/arm", line 6, in <module>
    exec(compile(open(__file__).read(), __file__, 'exec'))
  File "/Users/marca/dev/git-repos/ansible-role-manager/bin/arm", line 5, in <module>
    main()
  File "/Users/marca/dev/git-repos/ansible-role-manager/arm/main.py", line 48, in main
    args.func(args)
  File "/Users/marca/dev/git-repos/ansible-role-manager/arm/commands/freeze.py", line 19, in run
    _roles_directory = os.path.join(_root, 'roles')
  File "/Users/marca/python/virtualenvs/smstack/bin/../lib/python2.7/posixpath.py", line 77, in join
    elif path == '' or path.endswith('/'):
AttributeError: 'bool' object has no attribute 'endswith'

we get:

$ arm freeze
can't find playbook.
use `arm init` to create recommended structure.