Improve exception transparency: explicitly allow throwing exceptions …

[qwwdfsad]

…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

[qwwdfsad]

I've changed the contract, so the

 val flow = flow {
     try {
         emit(1)
     } finally {
         throw TestException()
     }
 }.catch { expectUnreached() }.onEach { throw TestException2() }

and

 val flow = flow {
     try {
         emit(1)
     } finally {
         throw TestException()
     }
 }.onEach { throw TestException2() }

now throw the same exception. Please re-review it carefully one more time

[dkhalanskyjb]

All tests are for the situation where upstream fails with TestException; nothing checks that the downstream exception is chosen if the upstream throws a CancellationException.