Expand retry on common connection errors

[carterm]

fetch-retry is doing a great job of resubmitting requests. Let's check to make sure other common connection errors are retried.

• 502 - Bad Gateway
• 504 - Target not responding

Also important to check for response codes before reading the JSON body...

FetchError: invalid json response body at https://dev-cagov-dcc.pantheonsite.io/wp-json/wp/v2/media?per_page=1&orderby=modified&order=desc&_fields=modified&cachebust=0.9978039968360015 reason: Unexpected token W in JSON at position 2

[carterm]

The checking for response codes before body will have to be done on a case-by-case.

[carterm]

The line in question is already checking for an OK response, but the content is coming back non-json. Likely related to retry preventable errors.