`cd foo/bar` where no "foo" dir exists crashes mantash

[trentm]

[trent.mick@manta /trent.mick/stor]$ cd asdf/asdf
mantash: ERROR: (ResourceNotFound) /trent.mick/stor/asdf was not found (/Users/trentm/joy/python-manta/manta/client.py:250 in list_directory2)

Traceback (most recent call last):
  File "/Users/trentm/joy/python-manta/bin/mantash", line 1895, in <module>
    retval = main(sys.argv)
  File "/Users/trentm/joy/python-manta/bin/mantash", line 1889, in main
    return shell.main(argv, loop=cmdln.LOOP_IF_EMPTY)
  File "/Users/trentm/joy/python-manta/manta/cmdln.py", line 268, in main
    return self.cmdloop()
  File "/Users/trentm/joy/python-manta/manta/cmdln.py", line 358, in cmdloop
    retval = self.onecmd(argv)
  File "/Users/trentm/joy/python-manta/manta/cmdln.py", line 422, in onecmd
    return self._dispatch_cmd(handler, argv)
  File "/Users/trentm/joy/python-manta/manta/cmdln.py", line 1125, in _dispatch_cmd
    return handler(argv[0], opts, *args)
  File "/Users/trentm/joy/python-manta/bin/mantash", line 652, in do_cd
    dirents = self.client.ls(parent)
  File "/Users/trentm/joy/python-manta/manta/client.py", line 687, in ls
    mdir, marker=marker)
  File "/Users/trentm/joy/python-manta/manta/client.py", line 250, in list_directory2
    raise errors.MantaAPIError(res, content)
MantaAPIError: (ResourceNotFound) /trent.mick/stor/asdf was not found