internal/worker: retry shedded fetches
internal/worker: retry shedded fetches

If a fetch returns a 503 (ServiceUnavailable), it was prevented from
running because of load shedding. We want the task queue to retry
that module for us.

Change-Id: I4f306a259e1cf30cf21e78f06ee4145ef5941aeb
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/257177
Trust: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>
Run-TryBot: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>
TryBot-Result: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Julie Qiu <8d32267b6b4884cf35adeaccde2b6857ae11aace@golang.org>

golang/pkgsite,golang/pkgsite,golang/pkgsite,golang/pkgsite