Add backoff values for Asus store

[judy]

Description

When including Asus in the list of stores, streetmerchant would successfully check the site 1-3 times, before repeatedly getting 503 status codes. I think it's running up against a rate limiter.

Possible solution

I'm submitting a PR now that adds the usual backoff codes, plus 503, so that streetmerchant can still successfully check the Asus store. I've tried this and it works great.

[judy]

PR: #790