[plt-scheme] Snooze and application-level validation before persisting

I've been looking at the Snooze documentation

http://planet.plt-scheme.org/package-source/untyped/snooze.plt/2/2/planet-docs/snooze/index.html

I don't see anything about validating data before attempting to
persist it to the database. I'm curious about how a programmer using
Snooze would go about validating in the application. If there is
invalid data, how does the model layer then either throw errors or
return an errors object rather than attempting to persist the data.

I'm thinking of a web application where the errors then need to be
displayed to the user in an HTML form but this is not really a
web-specific issue.

Thanks,
Peter