Investigate Sentry error: RequestGenerationFailed in WebPush

[callahad]

Our monitoring for Syngal on matrix.org occasionally sees an error:

RequestGenerationFailed: [<twisted.python.failure.Failure builtins.AttributeError: 'FileBodyProducer' object has no attribute '_task'>]
  File "sygnal/http.py", line 265, in _handle_dispatch
    result = await pushkin.dispatch_notification(notif, d, context)
  File "sygnal/notifications.py", line 180, in dispatch_notification
    return await self._dispatch_notification_unlimited(n, device, context)
  File "sygnal/webpushpushkin.py", line 183, in _dispatch_notification_unlimited
    response = await response_wrapper.deferred
  File "twisted/internet/defer.py", line 749, in send
    raise result.value

So... what causes this to happen? Can we handle the error more gracefully?

(For Element employees, this is https://sentry.matrix.org/sentry/sygnal-matrixorg/issues/216317/)