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?
We should let any other pipeline running finish first though.