[Worker V2] Fail gracefully on invalid http output

[mhoeger]

Fail gracefully on invalid output for http response. Especially because breaking change, need very explicit error message.
#264

Currently, this:

Exception: TypeError: Cannot create property 'headers' on string 'http output'
Stack: TypeError: Cannot create property 'headers' on string 'http outpu'
    at Object.toRpcHttp [as converter] (D:\home\SiteExtensions\Functions\32bit\workers\node\worker-bundle.js:19813:25)
    at resultCallback (D:\home\SiteExtensions\Functions\32bit\workers\node\worker-bundle.js:17876:70)
    at InvocationContext.done (D:\home\SiteExtensions\Functions\32bit\workers\node\worker-bundle.js:19685:13)
    at D:\home\SiteExtensions\Functions\32bit\workers\node\worker-bundle.js:17915:47
    at processTicksAndRejections (internal/process/task_queues.js:93:5)

Should check if output is object or not and give appropriate recommendation.