Improve the type safety of errorinfo.ReportException.

[junkmd]

Due to the possibility that sys.exc_info()[2] and TracebackType.tb_next may return None, there are points in errorinfo.ReportException where the type checker will produce errors.

If this function is called with a specified stacklevel without catching an error, or if stacklevel is specified deeper than the cause of the error even when an error is caught, an AttributeError will be raised.

By explicitly detecting such cases and throwing a ValueError, the implementation of errorinfo.ReportException can be made type-safe.