sweet/cmd/sweet: fix wrong return value

The return value err here must be nil. Considering the context, readErr should be returned here.

Change-Id: Icd063d54a490440189066af0a87c0c2b372b8af8
Reviewed-on: https://go-review.googlesource.com/c/benchmarks/+/653835
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>
Auto-Submit: Carlos Amedee <carlos@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>