Nicer error when cwd is not playbook dir
Nicer error when cwd is not playbook dir

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.

devekko/ansible-role-manager,mirskytech/ansible-role-manager,devekko/ansible-role-manager,mirskytech/ansible-role-manager