return read error on non EAGAIN errno

return read error on non EAGAIN errno

Summary: Tested over weekend with D2571554. There was no EXC_RESOURCE/CPU crash when read errrors were returned on ENOTCONN. EAGAIN seems innocent as the spin detector was disabled on testing group.

Patch folly. I can add #ifdef __APPLE__ around the errno checking code but this should be good practice for non apple code as well.

I will remove debugging code in fbios master in another diff.

Reviewed By: djwatson

Differential Revision: D2580819

fb-gh-sync-id: 9162a3deba01af8b07cd2b336d7da3da040c67a9