make Extract propagate errors from its fstream writer

Extract contains an fstream Writer but never listens for error events, so the creator of the Extract object cannot catch those errors without reaching into _fst which seems like a bad idea..  this change just propagates those errors so the caller can listen for errors on extract.

I would add a unit test but I'm unsure how to reliably create an error from the Writer (I ran across this with ENOSPC which isn't easy to re-create on demand)