Summary: deferError adds an error handling callback to SemiFuture, enqueued on the internal deferred executor, and thus will run on the chained executor or inline with .get() as for defer.

Reviewed By: yfeldblum

Differential Revision: D7243193

fbshipit-source-id: 9fe7dab9fbc8b236d717b058c81a303d87fa9253