all.bash: change staticcheck errors to err level

When a staticcheck error occurs, upgrade it to an error so that CI
will fail for the change.

Change-Id: Ied94c9829eed6863b4fc5a1d4c4bb979a7c06c27
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/281993
Trust: Julie Qiu <julie@golang.org>
Run-TryBot: Julie Qiu <julie@golang.org>
TryBot-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Jonathan Amsterdam <jba@google.com>
diff --git a/all.bash b/all.bash
index 75c2343..fd14a35 100755
--- a/all.bash
+++ b/all.bash
@@ -100,7 +100,7 @@
 # check_staticcheck runs staticcheck on source files.
 check_staticcheck() {
   ensure_go_binary honnef.co/go/tools/cmd/staticcheck
-  runcmd staticcheck $(go list ./... | grep -v third_party | grep -v internal/doc | grep -v internal/render) | warnout
+  runcmd staticcheck $(go list ./... | grep -v third_party | grep -v internal/doc | grep -v internal/render)
 }
 
 # check_misspell runs misspell on source files.