sweet: print additional details during a subprocess failure

This change also updates the places that print errors to use log.Error
instead of rolling their own methods.

Change-Id: I4a858f43213c4cd9ade1d777795f380977b87f1b
Reviewed-on: https://go-review.googlesource.com/c/benchmarks/+/383536
Trust: Michael Knyszek <mknyszek@google.com>
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Pratt <mpratt@google.com>
3 files changed