cmd/bent: fixed error handling in docopy closure.

Change-Id: Icb6f96ede3bda73a026f8037e3be80f608e9df4b
Reviewed-on: https://go-review.googlesource.com/c/benchmarks/+/674375
Reviewed-by: David Chase <drchase@google.com>
Auto-Submit: David Chase <drchase@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>