RouterException: Server error: NOT_FOUND: Requested entity was not found.

[jrconlin]

FCM returns a "NOT_FOUND" error if a device is no longer present/ not found. This should return a 404.

https://sentry.prod.mozaws.net/operations/autopush-prod/issues/6072684/

RouterException: Server error: NOT_FOUND: Requested entity was not found.
  File "twisted/internet/defer.py", line 654, in _runCallbacks
    current.result = callback(current.result, *args, **kw)
  File "autopush/router/fcmv1client.py", line 122, in error
    raise RouterException("Server error: {}".format(failure.value))

RouterException: Server error: NOT_FOUND: Requested entity was not found.