error in map fn propagates downstream
error in defer fn propagates downstream
error in flatMap fn propagates downstream
error in recover fn propagates downstream
error in then fn propagates downstream
conform terminal error handling
fix async recover downstream test
handle terminal continuation errors
javadoc continuation error behaviors

- fix how then retry fn err is handled
defer-x
foreach-x and on-x
refactor for each, on, defer
map-x and flatmap-x
recover-x
then-x
collapse API
simplify fun-x interfaces
Merge branch 'exceptions'