poor error message when interaction protocol fails

[rogpeppe]

When an interaction fails, the bakery only prints the HTTP status code, not the actual returned error.
For example:

Traceback (most recent call last):
  File "/snap/canonical-rbac/18/bin/crbs-admin", line 11, in <module>
    load_entry_point('crbs==1.0.0', 'console_scripts', 'crbs-admin')()
  File "/snap/canonical-rbac/18/lib/python3.6/site-packages/crbs/cmd/_script.py", line 60, in __call__
    return self._actions[options.action](options)
  File "/snap/canonical-rbac/18/lib/python3.6/site-packages/crbs/cmd/_script.py", line 18, in __call__
    self.execute(options)
  File "/snap/canonical-rbac/18/lib/python3.6/site-packages/crbs/cmd/admin/_create_candid_agent.py", line 49, in execute
    parent=True)
  File "/snap/canonical-rbac/18/lib/python3.6/site-packages/crbs/candid.py", line 146, in create_user
    response = self._request('POST', 'v1/u', json=content)
  File "/snap/canonical-rbac/18/lib/python3.6/site-packages/crbs/candid.py", line 162, in _request
    resp = self._client.request(method, f'{self.url}/{path}', json=json)
  File "/snap/canonical-rbac/18/lib/python3.6/site-packages/macaroonbakery/httpbakery/_client.py", line 87, in request
    return requests.request(method=method, url=url, **kwargs)
  File "/snap/canonical-rbac/18/lib/python3.6/site-packages/requests/api.py", line 60, in request
    return session.request(method=method, url=url, **kwargs)
  File "/snap/canonical-rbac/18/lib/python3.6/site-packages/requests/sessions.py", line 533, in request
    resp = self.send(prep, **send_kwargs)
  File "/snap/canonical-rbac/18/lib/python3.6/site-packages/requests/sessions.py", line 653, in send
    r = dispatch_hook('response', hooks, r, **kwargs)
  File "/snap/canonical-rbac/18/lib/python3.6/site-packages/requests/hooks.py", line 31, in dispatch_hook
    _hook_data = hook(hook_data, **kwargs)
  File "/snap/canonical-rbac/18/lib/python3.6/site-packages/macaroonbakery/httpbakery/_client.py", line 292, in hook
    client.handle_error(error, req.url)
  File "/snap/canonical-rbac/18/lib/python3.6/site-packages/macaroonbakery/httpbakery/_client.py", line 104, in handle_error
    self.key,
  File "/snap/canonical-rbac/18/lib/python3.6/site-packages/macaroonbakery/bakery/_discharge.py", line 77, in discharge_all
    dm = get_discharge(cav.cav, cav.encrypted_caveat)
  File "/snap/canonical-rbac/18/lib/python3.6/site-packages/macaroonbakery/httpbakery/_client.py", line 156, in acquire_discharge
    'discharge failed with code {}'.format(resp.status_code))
macaroonbakery.httpbakery._error.DischargeError: third party refused dischargex: discharge failed with code 500

The code should treat the error response from a discharge endpoint as a regular error and print the message contained in it.