Return error code if report contains error

If a task fails for any file, I think plumber-cli should return an error exit code. This is useful when Plumber is invoked from a script, e.g. `plumber jshint` when used in a Git pre-commit hook.

We output the reports here: https://github.com/plumberjs/plumber/blob/master/lib/operation/output-reports.js. We could have another operation that looks for any error if a report and returns an error exit code if it finds one.

Thoughts?
Ahh, the CLI could be running multiple pipelines, each with its own execution. I see now.

I think my solution to this may be different if we solve https://github.com/plumberjs/plumber-cli/pull/5 first.