fix await multithreaded data race

coro return was reading from a value that coro await was
writing to. that wasn't how it was designed to work, it
was an implementation mistake.

this commit also has some work-in-progress code for fixing
error return traces across suspend points.
84e952c230Andrew Kelley committed on 3/10/2018, 6:38:40 AMparent3b3649b
6 files changedLine totals unavailable