Handling errors in onSync causes infinite loop.

[chimon2000]

Library Affected:
workbox-background-sync

Browser & Platform:
Google Chrome v72.0

Issue or Feature Request Description:
I am attempting to override the onSync method such that failing status codes add the request back to the queue:

                const response = await fetch(entry.request.clone())

                if ([504, 500].includes(response.status)) {
                    throw new Error('Cannot reach server')
                }

This results in an infinite loop of syncs to occur.

[chimon2000]

I believe this happens because adding a request back to the queue causes a new sync to be triggered.

workbox/packages/workbox-background-sync/Queue.mjs

Lines 182 to 183 in bd5f411

	await this._queueStore[`${operation}Entry`](entry);
	await this.registerSync();

[jeffposnick]

CC: @philipwalton for visibility.