Handle python's 120 exit code

[sgillies]

See https://docs.python.org/3/library/sys.html#sys.exit. 120 is the python interpreter's only special code.

This works around issue #339. There's a deeper problem to be solved, but even so this is a useful patch. We never want to retry in the case that the worker gets a 120 exit code from python.

[rclark]

Could you add a test for this exit code, similar to this one?

ecs-watchbot/test/worker.test.js

Lines 311 to 336 in c98f0c2

	test('[worker] waitFor, exit 4', async (assert) => {
	const logger = stubber(Logger).setup();
	logger.log.restore();
	logger.stream.restore();
	const message = sinon.createStubInstance(Message);
	message.env = { Message: 'banana', SentTimestamp: '2019-02-09T21:57:55.000Z' };
	const options = { command: 'exit 4', volumes: ['/tmp'] };
	const worker = new Worker(message, options);
	sinon.spy(child_process, 'spawn');
	try {
	await worker.waitFor();
	} catch (err) {
	assert.ifError(err, 'failed');
	}
	const results = logger.workerSuccess.args[0][0];
	assert.equal(results.code, 4, 'logged worker success exit code');
	assert.ok(results.duration, 'logged worker success duration');
	assert.ok(results.response_duration, 'logged worker response duration');
	assert.equal(message.retry.callCount, 1, 'calls message.retry()');
	child_process.spawn.restore();
	logger.teardown();
	assert.end();

[sgillies]

@rclark ok, test added.

[rclark]

yummy