[issue41756] Do not always use exceptions to return result from coroutine

[Vladimir Matveev]

Vladimir Matveev <desco.by at gmail.com> added the comment:

If I understand proposed shape of API correctly - it was not supposed to return exception via "result" so contract for new `PyGen_Send` function is something like:

Return value | result   | Comment
-----------------------------------------------------
PYGEN_RETURN | not NULL | Returned value
PYGEN_NEXT   | not NULL | Yielded value
PYGEN_ERROR  | NULL     | Regular PyErr_* functions should be used to work with error case

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue41756>
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