Handle async function output and return error in knative runtime #42

Try to resolve this: #10 (comment)

One more thing, I think go ff's handling of function output and return error is more reasonable.
What if send fails? Here're more refs:

• automatically set http ret code based on function error code https://github.com/OpenFunction/functions-framework-go/blob/main/runtime/knative/knative.go#L66
• encapsule function output into context: OpenFunction/functions-framework-go@f1e311b/runtime/runtime.go#L134
• return error in function when send fails or other errors occur : https://github.com/OpenFunction/samples/blob/main/functions/knative/with-output-binding/sender.go#L23