Graphsync does not return an error when the request context is cancelled

[dirkmc]

We found a problem in go-data-transfer whereby when the node shuts down

• the context for all the outstanding requests are cancelled
• go-data-transfer's graphsync transport consumes all outstanding requests
• the graphsync request completes without returning an error
• the transfer is marked as complete, even though it is incomplete

We worked around this problem by checking the if the request context was closed when the request finishes: filecoin-project/go-data-transfer#134. The correct solution would be to fix the underlying problem, which appears to be that graphsync does not return an error when the request context is cancelled.

[hannahhoward]

This is no longer true I think