Error not meaningful if glance-registry service not running

If the glance-api service is running but the glance-registry service is not, the user gets the following error message:

$ glance index
Failed to show index. Got error:
The request returned 500 Internal Server Error

The response body:
Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/eventlet/wsgi.py", line 336, in handle_one_response
    result = self.application(self.environ, start_response)
  File "/usr/lib/python2.7/dist-packages/webob/dec.py", line 147, in __call__
    resp = self.call_func(req, *args, **self.kwargs)
  File "/usr/lib/python2.7/dist-packages/webob/dec.py", line 210, in call_func
    return self.func(req, *args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/glance/common/wsgi.py", line 279, in __call__
    response = req.get_response(self.application)
  File "/usr/lib/python2.7/dist-packages/webob/request.py", line 1086, in get_response
    application, catch_exc_info=False)
  File "/usr/lib/python2.7/dist-packages/webob/request.py", line 1055, in call_application
    app_iter = application(self.environ, start_response)
  File "/usr/lib/python2.7/dist-packages/keystone/middleware/auth_token.py", line 176, in __call__
    return self.app(env, start_response)
  File "/usr/lib/python2.7/dist-packages/webob/dec.py", line 147, in __call__
    resp = self.call_func(req, *args, **self.kwargs)
  File "/usr/lib/python2.7/dist-packages/webob/dec.py", line 210, in call_func
    return self.func(req, *args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/glance/common/wsgi.py", line 279, in __call__
    response = req.get_response(self.application)
  File "/usr/lib/python2.7/dist-packages/webob/request.py", line 1086, in get_response
    application, catch_exc_info=False)
  File "/usr/lib/python2.7/dist-packages/webob/request.py", line 1055, in call_application
    app_iter = application(self.environ, start_response)
  File "/usr/lib/python2.7/dist-packages/webob/dec.py", line 159, in __call__
    return resp(environ, start_response)
  File "/usr/lib/python2.7/dist-packages/routes/middleware.py", line 131, in __call__
    response = self.app(environ, start_response)
  File "/usr/lib/python2.7/dist-packages/webob/dec.py", line 159, in __call__
    return resp(environ, start_response)
  File "/usr/lib/python2.7/dist-packages/webob/dec.py", line 147, in __call__
    resp = self.call_func(req, *args, **self.kwargs)
  File "/usr/lib/python2.7/dist-packages/webob/dec.py", line 210, in call_func
    return self.func(req, *args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/glance/common/wsgi.py", line 477, in __call__
    request, **action_args)
  File "/usr/lib/python2.7/dist-packages/glance/common/wsgi.py", line 494, in dispatch
    return method(*args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/glance/api/v1/images.py", line 134, in index
    images = registry.get_images_list(req.context, **params)
  File "/usr/lib/python2.7/dist-packages/glance/registry/__init__.py", line 129, in get_images_list
    return c.get_images(**kwargs)
  File "/usr/lib/python2.7/dist-packages/glance/registry/client.py", line 77, in get_images
    res = self.do_request("GET", "/images", params=params)
  File "/usr/lib/python2.7/dist-packages/glance/common/client.py", line 58, in wrapped
    return func(self, *args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/glance/common/client.py", line 420, in do_request
    headers=headers)
  File "/usr/lib/python2.7/dist-packages/glance/common/client.py", line 75, in wrapped
    return func(self, method, url, body, headers)
  File "/usr/lib/python2.7/dist-packages/glance/common/client.py", line 557, in _do_request
    raise exception.ClientConnectionError(e)
ClientConnectionError: There was an error connecting to a server
Details: [Errno 111] ECONNREFUSED

It would be much more helpful if the error was something that more explicitly described the problem to the user, such as:

$ glance index
ERROR: The glance-api service was not able to connect to the glance-registry service.
Possible sources of error:
 * the glance-registry service is not running
 * In glance-api.conf, the registry_host or registry_port settings are incorrect