sweet: make it clear which benchmarks failed

Currently if there's a failure, you just get a message like "benchmarks
failed to execute" but that's not very specific. It can make recent logs
look like the culprit, when really it's further up in the log.

Change-Id: Ib6ac96335b00418041495e5d1f9a6e2db920d387
Reviewed-on: https://go-review.googlesource.com/c/benchmarks/+/614315
Reviewed-by: Michael Pratt <mpratt@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>