novatools should handle exceptions more cleanly

For example, when exceeding a quota I get this stacktrace:

    $ novatools boot win102 --image 4 --flavor 3
    Traceback (most recent call last):
      File "/usr/local/bin/novatools", line 9, in <module>
        load_entry_point('python-novatools==1.2', 'console_scripts', 'novatools')()
      File "/usr/local/lib/python2.6/dist-packages/python_novatools-1.2-py2.6.egg/novatools/shell.py", line 593, in main
        OpenStackShell().main(sys.argv[1:])
      File "/usr/local/lib/python2.6/dist-packages/python_novatools-1.2-py2.6.egg/novatools/shell.py", line 144, in main
        args.func(args)
      File "/usr/local/lib/python2.6/dist-packages/python_novatools-1.2-py2.6.egg/novatools/shell.py", line 284, in do_boot
        metadata, files)
      File "/usr/local/lib/python2.6/dist-packages/python_novatools-1.2-py2.6.egg/novatools/servers.py", line 222, in create
        return self._create("/servers", body, "server")
      File "/usr/local/lib/python2.6/dist-packages/python_novatools-1.2-py2.6.egg/novatools/base.py", line 34, in _create
        resp, body = self.api.client.post(url, body=body)
      File "/usr/local/lib/python2.6/dist-packages/python_novatools-1.2-py2.6.egg/novatools/client.py", line 87, in post
        return self._cs_request(url, 'POST', **kwargs)
      File "/usr/local/lib/python2.6/dist-packages/python_novatools-1.2-py2.6.egg/novatools/client.py", line 71, in _cs_request
        **kwargs)
      File "/usr/local/lib/python2.6/dist-packages/python_novatools-1.2-py2.6.egg/novatools/client.py", line 57, in request
        raise exceptions.from_response(resp, body)
    novatools.exceptions.OpenStackException: InstanceLimitExceeded: Instance quota exceeded. You can only run 0 more instances of this type. (HTTP 500)

The tool should probably handle any exception due to interacting with the API and print a clean error message to the user. The message from the exception would probably be good.