code: def addErrback(self, errback, *args, **kw):
          
          return self.addCallbacks(
              _passthrough, errback, errbackArgs=args, errbackKeywords=kw)
docs: Add a failure callback that will be run in the context of an eliot
          action.
  
          @return: C{self}
          @rtype: L{DeferredContext}
  
          @raises AlreadyFinished: L{DeferredContext.addActionFinish} has been
              called. This indicates a programmer error.