Feature/pub 71 add error handler

[ketan-saxena]

PUB-71:

• Add handleError in Subscription class
• Add handleError method in SubscriberOptions for SubscriptionService
• Update example subscription

[rohit-gohri]

LGTM with some nitpicks regarding defaults

[rohit-gohri]

Let's rethrow the error in the default handler, so application can handle it on their own

Suggested change

	public static handleError(error: Error): void {
	// default error handling logic
	Logger.Instance.error({ error }, 'Received Unexpected Error');
	}
	public static handleError(error: Error): void {
	// default error handling logic
	Logger.Instance.error({ error }, 'Received Unexpected Error');
	throw error;
	}

[rohit-gohri]

Let's add an example similar to google docs: https://googleapis.dev/nodejs/pubsub/latest/index.html#using-the-client-library

Suggested change

	handleError: function(error: Error): void {
	// internal error handling logic for subscriber
	}
	handleError: function(error: Error): void {
	// internal error handling logic for subscriber
	console.error('Subscription error', error);
	// shutdown any connections here and exit
	process.exit(1);
	}

Similar handlers in other examples also