Better error message for "incomplete response"

[dirkmc]

Currently when the data transfer fails, eg because the miner crashes, the client receives the message "incomplete response".
This comes from OnChannelCompleted() https://github.com/filecoin-project/go-data-transfer/blob/master/impl/events.go#L319

We should provide an error message that is more human-readable, like "the sender aborted the data transfer"

Example where this has caused confusion: filecoin-project/lotus#5409 (comment)