Combinators and fail-fast exceptions

[rnarubin]

Porting issue 46 from the enterprise repo

rnarubin commented May 2, 2018

Currently the operations in Combinators wait for all of their constituent stages to complete before completing the resulting stage. There could be cases, however, when a user wants to complete as soon as an exception is encountered (analogous to throwing an exception in the middle of a loop). We should add such fail-fast methods