Build fails but binderbot still tries to start kernel

[rabernat]

This run shows a case where binder failed to build the image, but then proceeded to try to start the kernel:
https://github.com/pangeo-gallery/cmip6/runs/629889320?check_suite_focus=true

2020-04-29 14:11.50 Binder: Waiting on event stream (phase: building) action=binder-start phase=event-stream
2020-04-29 14:11.50 Binder: Waiting on event stream (phase: failure) action=binder-start phase=event-stream
2020-04-29 14:11.50 Binder: Waiting on event stream (phase: Failed) action=binder-start phase=event-stream
2020-04-29 14:11.50 Binder: Waiting on event stream (phase: Failed) action=binder-start phase=event-stream
2020-04-29 14:11.50 Binder: Waiting on event stream (phase: built) action=binder-start phase=event-stream
2020-04-29 14:12.50 Kernel: Starting               action=kernel-start phase=start
2020-04-29 14:12.50 Kernel: Start failed 'BinderUser' object has no attribute 'token' action=kernel-start duration=8.800000017572529e-06 phase=failed
/opt/hostedtoolcache/Python/3.8.2/x64/lib/python3.8/site-packages/binderbot/cli.py:14: DeprecationWarning: "@coroutine" decorator is deprecated since Python 3.8, use "async def" instead
  f = asyncio.coroutine(f)
Traceback (most recent call last):
  File "/opt/hostedtoolcache/Python/3.8.2/x64/lib/python3.8/site-packages/binderbot/binderbot.py", line 109, in start_kernel
    headers = {'Authorization': f'token {self.token}'}
AttributeError: 'BinderUser' object has no attribute 'token'

During handling of the above exception, another exception occurred:

This should not happen. Instead, binderbot should quit after failing to build the image.