asyncio: Fix _ProactorWritePipeTransport._pipe_closed()
asyncio: Fix _ProactorWritePipeTransport._pipe_closed()

The "exc" variable was not defined, pass a BrokenPipeError exception instead.

Martiusweb/asyncio,haypo/trollius,gvanrossum/asyncio,gvanrossum/asyncio,gsb-eng/asyncio,Martiusweb/asyncio,gsb-eng/asyncio,vxgmichel/asyncio,1st1/asyncio,fallen/asyncio,ajdavis/asyncio,1st1/asyncio,manipopopo/asyncio,vxgmichel/asyncio,gvanrossum/asyncio,haypo/trollius,jashandeep-sohi/asyncio,haypo/trollius,Martiusweb/asyncio,1st1/asyncio,jashandeep-sohi/asyncio,ajdavis/asyncio,jashandeep-sohi/asyncio,manipopopo/asyncio,manipopopo/asyncio,gsb-eng/asyncio,ajdavis/asyncio,vxgmichel/asyncio,fallen/asyncio,fallen/asyncio