internal/worker: only report serious errors
internal/worker: only report serious errors

Send an error to the Error Reporting service only
if it is a true server error (code 500).

Change-Id: I3f4da6e8005ed29e6da4a25c4a234998f8db4b22
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/292930
Trust: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>
Run-TryBot: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>
TryBot-Result: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Jamal Carvalho <e0195770807aa8c82b0b128d9c0423b5ad035172@golang.org>

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