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?
Do we really want to exit immediately as soon as an error is encountered? This will also kill watch?