fix: return error if GetOperation call fails

[alexander-fenster]

Fixes googleapis/nodejs-video-intelligence#523.

When a long running operation starts, we start polling for result by sending GetOperation calls. Apparently, if a GetOperation call itself fails (e.g. with 8 RESOURCE_EXHAUSTED), this error is never handled properly, resulting in an unhandled rejection.

The problem is in the following piece of code:

gax-nodejs/src/longRunningCalls/longrunning.ts

Lines 192 to 196 in b01bf8d

	const noCallbackPromise = this.currentCallPromise_!.then(responses => {
	self.latestResponse = responses[0] as LROOperation;
	self._unpackResponse(responses[0] as LROOperation, callback);
	return promisifyResponse()!;
	});

In the created promise, the promisifyResponse call handles the case where GetOperation returned success but the operation itself has failed, but the case where GetOperation itself fails is not handled. I'm adding error handling in this place by supplying the second parameter to .then() and calling the provided callback if it's set, and rejecting the promise if not. In the real life use case, the callback is supplied:

gax-nodejs/src/longRunningCalls/longrunning.ts

Lines 293 to 297 in b01bf8d

	self.getOperation((err, result, metadata, rawResponse) => {
	if (err) {
	setImmediate(emit, 'error', err);
	return;
	}

I added a unit test for this fix, and tested it locally with the real quota errors returned by Video Intelligence API.

[alexander-fenster]

Just an update - this fix broke our integration tests, which may or may not mean that something else is broken. I'm looking deeper. The fun thing is that the integration tests worked (calls returned correct results), but some errors were not caught, and with this fix they are being caught now. I need to figure out if it's a bug in the tests, or in the main code.

[alexander-fenster]

This should go in after #1310 fixes the broken tests.