Ably-js does not call error callback when channel attach fails when using comet transport

[SimonWoolf]

With websocket transport, the attach callback is called with err = {"statusCode":400,"code":40010,"message":"Invalid channel id ()"}

With xhr, the callback is never called, though the error is logged to the console: "Ably: CometTransport.sendItems(): on complete: err = {"statusCode":400,"code":40010,"message":"Invalid channel id ()","serverId":"frontend.17d9.sandbox-eu-central-1-A.i-aaf1c864"}"

Example: channelattachempty test with transports: ['xhr']

[mattheworiordan]

Rerun this test when new transport upgrade changes are deployed

[SimonWoolf]

Confirm fixed by upgrade changes.