…from the upstream when the downstream has been failed, but also suppress such exceptions by the downstream one

It solves the problem of graceful shutdown: when the upstream fails unwillingly (e.g. file.close() has thrown in 'finally') block, we cannot treat is as an exception transparency violation (hint: 'finally' is a shortcut for 'catch'+body), but we also cannot leave things as is, otherwise it leads to unforeseen consequences such as successful 'retry' and 'catch' operators that may, or may not, then fail with exception on attempt to emit.

Downstream exception supersedes the upstream exception only if it is not an instance of CancellationException, semantically emulating cancellation-friendly 'use' block.

Fixes #2860

Co-authored-by: dkhalanskyjb <52952525+dkhalanskyjb@users.noreply.github.com>

All the benefits of the approach stay the same, but additionally, for an arbitrary flow pipeline, adding a catch operator that is not triggered will no longer change the type of resulting exception