Refine error handling in sample application

[sergefdrv]

Initially, request function would return an error that is propagated by cobra up the call stack, and finally reported here. Maybe we should restore that behavior (in a separate PR)?

In that case, it seems not necessary to output the error here, because sample/peer/cmd.Execute will do that. We might also want to report the error to stderr rather than stdout.

Originally posted by @sergefdrv in #134