The patch looks good to me, but I'd like to see the error message convey two points:
- the coroutine has already terminated (regardless of how that happened)
- the calling code attempted to resume it anyway

That is, something like "Cannot resume terminated coroutine", rather than specifically referring to "await".

After all, waiting for the result with "await" is only one way to terminate a coroutine - you can also get there with direct calls to next(), send(), throw() and close().