internal/fetch: include zip size in loadshed error message

Change-Id: Ida9d17c6b4e5b35a7566768a397bc57c77f691e9
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/257478
Trust: Jonathan Amsterdam <jba@google.com>
Run-TryBot: Jonathan Amsterdam <jba@google.com>
TryBot-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Julie Qiu <julie@golang.org>
diff --git a/internal/fetch/fetch.go b/internal/fetch/fetch.go
index 242a4b7..4b79b15 100644
--- a/internal/fetch/fetch.go
+++ b/internal/fetch/fetch.go
@@ -167,7 +167,7 @@
 	shouldShed, deferFunc := zipLoadShedder.shouldShed(uint64(zipSize))
 	fr.Defer = deferFunc
 	if shouldShed {
-		fr.Error = derrors.SheddingLoad
+		fr.Error = fmt.Errorf("%w: size=%dMi", derrors.SheddingLoad, zipSize/mib)
 		stats.Record(ctx, fetchesShedded.M(1))
 		return fr
 	}