subscription manager client reports errors pretty poorly

[exarkun]

For example, if the server returns a 500, the traceback the client logs is:

main function encountered error
Traceback (most recent call last):
  File "/home/exarkun/Environments/leastauthority.com/local/lib/python2.7/site-packages/twisted/internet/defer.py", line 565, in _startRunCallbacks
    self._runCallbacks()
  File "/home/exarkun/Environments/leastauthority.com/local/lib/python2.7/site-packages/twisted/internet/defer.py", line 651, in _runCallbacks
    current.result = callback(current.result, *args, **kw)
  File "/home/exarkun/Environments/leastauthority.com/local/lib/python2.7/site-packages/twisted/internet/defer.py", line 457, in callback
    self._startRunCallbacks(result)
  File "/home/exarkun/Environments/leastauthority.com/local/lib/python2.7/site-packages/twisted/internet/defer.py", line 565, in _startRunCallbacks
    self._runCallbacks()
--- <exception caught here> ---
  File "/home/exarkun/Environments/leastauthority.com/local/lib/python2.7/site-packages/twisted/internet/defer.py", line 651, in _runCallbacks
    current.result = callback(current.result, *args, **kw)
  File "/home/exarkun/Work/LeastAuthority/leastauthority.com/lae_automation/subscription_manager.py", line 468, in check
    raise UnexpectedResponseCode(response, required)
lae_automation.subscription_manager.UnexpectedResponseCode: 

I would expect UnexpectedResponseCode to have a better string representation than that, given it is defined with attr. So I'm not quite sure what's happening here.