internal/frontend: fix 400 handling in checkForPath

checkForPath now returns a 404 for 400s, instead of ignoring this state.

Change-Id: I50898380574ce40dba30b21a2ec2e109c06c0acf
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/279724
Reviewed-by: Jonathan Amsterdam <jba@google.com>
Trust: Julie Qiu <julie@golang.org>
diff --git a/internal/frontend/fetch.go b/internal/frontend/fetch.go
index fce071e..556cc53 100644
--- a/internal/frontend/fetch.go
+++ b/internal/frontend/fetch.go
@@ -399,7 +399,7 @@
 		}
 		// All remaining non-200 statuses will be in the 40x range.
 		// In that case, just return a not found error.
-		if fr.status > 400 {
+		if fr.status >= 400 {
 			fr.status = http.StatusNotFound
 			fr.err = errModuleDoesNotExist
 			return