CLI: return a nonzero status when validation fails

[lwille]

This tool is great, but so far int merely prints errors and warnings, which makes it difficult to evaluate the validation status e.g. by a CI script or pre-commit hook.

A naïve implementation might just evaluate the list of errors and warnings and exit the process with a non-zero status:

    const errorsAndWarnings = validateSchema(targetObj, options)
    if (errorsAndWarnings?.length > 0) {
      process.exit(1)
    }

[ketanSaxena]

@lwille Looks like a useful improvement. But would like to have it added as an optional param that can be passed if we want to exit process on errors/warnings. That way the changes in package would be backward compatible.