如何在不出错的情况下测试Twisted Deferred error呢?

I have some Twisted code which creates multiple chains of Deferreds. Some of these may fail without having an errback which puts them back on the callback chain. I haven't been able to write a unit test for this code - the failing Deferred causes the test to fail after the test code has completed. How can I write a passing unit test for this code? Is it expected that every Deferred which could fail in normal operation should have an errback at the end of the chain which puts it back on the callback chain?I have some Twisted code which creates multiple