Issue2221: in Idle, exec('xx') raised a SystemError('error return without exception...

Issue2221: in Idle, exec('xx') raised a SystemError('error return without exception set')
instead of the expected NameError

This happens when sys.stdout is redirected to something that cannot flush().
the flush_io() function must be exception-neutral: don't raise, and don't clear exceptions.

Next step: exec() is not supposed to flush sys.stdout...