Improve coroutine exception handling logic

[qwwdfsad]

• Wrap exceptions into CE if parent cancels its child. This is possible because now we can be sure that no one is invoking 'cancel(Throwable)', thus if parent cancels its children, an exception is definitely handled.
• This changes allows exceptions in hierarchies to be handled only once even when coroutines in the hierarchy are capable of handling exceptions
• Breaking change: CompletableJob.completeExceptionally(exception) uses its parameter as additional debug information for children ('cause' of CE), thus changing the result of its children. For example, if CJ was a parent of CompleteableDeferred and CJ was completed exceptionally with exception E1, now CD.await() will throw CE instead of E1
• Recursively check whether parent handles exception to avoid duplicates reporting when Job() is present in the hierarchy

Fixes for #689

[elizarov]

Good to go with one small improvement