internal/middleware: don't generate an error report for load shedding
internal/middleware: don't generate an error report for load shedding

The worker sheds load by returning a 503. That is a normal action,
not a true application error, so don't contact the error-reporting
API.

Change-Id: Ie83b4abe65e4523a466c68552bc58dff1aae638c
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/268637
Trust: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>
Run-TryBot: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>
Reviewed-by: Robert Findley <9179eee875d1ee628c37fb47d7e749e855ffd21f@google.com>
Reviewed-by: Jamal Carvalho <e0195770807aa8c82b0b128d9c0423b5ad035172@golang.org>
Reviewed-by: Julie Qiu <8d32267b6b4884cf35adeaccde2b6857ae11aace@golang.org>
TryBot-Result: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

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