Throw Error if lowstates aren't a list (instead of stack tracing). Old trace was:

```
return: "Traceback (most recent call last):\n  File \"/usr/lib/python2.6/site-packages/saltapi/netapi/rest_cherrypy/app.py\"\
  , line 255, in hypermedia_handler\n    ret = cherrypy.serving.request._hypermedia_inner_handler(*args,\
  \ **kwargs)\n  File \"/usr/lib/python2.6/site-packages/cherrypy/_cpdispatch.py\"\
  , line 34, in __call__\n    return self.callable(*self.args, **self.kwargs)\n  File\
  \ \"/usr/lib/python2.6/site-packages/saltapi/netapi/rest_cherrypy/app.py\", line\
  \ 532, in POST\n    'return': list(self.exec_lowstate()),\n  File \"/usr/lib/python2.6/site-packages/saltapi/netapi/rest_cherrypy/app.py\"\
  , line 432, in exec_lowstate\n    chunk['token'] = token\nTypeError: 'unicode' object\
  \ does not support item assignment\n"
status: 500

```