As simply a front controller implementation, Front Man doesn't have any error handling built in so that you can use whatever works best for your app.
For example, in the Web 2.0 (I hate that term!) application that I'm currently working on, almost all form submissions happen through Ajax, and errors are returned as a JSON serialization that is then integrated with the jQuery Validation plugin for display.
Obviously that's not appropriate for all apps.
For a canonical Model-2 web app, I used to tack a "message list" object onto the session (used the PRG pattern, so can't simply carry it on the request) for display on the page in the event of error. The list would be removed form the session upon display.
So, the best way for your app to carry error messages to the view depends upon how it's structured. Does your login command forward back to the form on error? If so, carrying the message(s) on the request should work fine. If a redirect, the session might work better (with appropriate cleanup code). Under Ajax, a JSON response works great. And so on...