Intermittent error handling in poll

[krasnoukhov]

We've been getting a following message running this package in production:

Forbidden [no error message] 
    node_modules/aws-sdk/lib/services/s3.js:557:35 Request.extractError
    node_modules/aws-sdk/lib/sequential_executor.js:105:20 Request.callListeners
    node_modules/aws-sdk/lib/sequential_executor.js:77:10 Request.emit
    node_modules/aws-sdk/lib/request.js:683:14 Request.emit
    node_modules/aws-sdk/lib/request.js:22:10 Request.transition
    node_modules/aws-sdk/lib/state_machine.js:14:12 AcceptorStateMachine.runTo
    node_modules/aws-sdk/lib/state_machine.js:26:10 
    node_modules/aws-sdk/lib/request.js:38:9 Request.<anonymous>
    node_modules/aws-sdk/lib/request.js:685:12 Request.<anonymous>
    node_modules/aws-sdk/lib/sequential_executor.js:115:18 Request.callListeners

It seems to be very flaky. Usually it starts to appear after fastboot has been running for a while with no restarts. I suspect it could be due to some network issues.

However, looking at the current poll code:

fastboot-s3-notifier/index.js

Lines 49 to 55 in 6dafdf1

	poll() {
	this.s3.headObject(this.params).promise()
	.then(data => {
	this.compareLastModifieds(data.LastModified);
	this.schedulePoll();
	});
	}

It does not seem like errors are handled. So if headObject fails the notifier will stop the polling, effectively preventing future retries. This causes app to not update even if new version is deployed.