Clean up validation error messages

[ejmg]

Summary

Error messages from Zod (and, by extension, those returned by API responses) are not useful in their current state.

Description

Part of why #25 exists is that Zod's error message facilities are not being leveraged; instead, auto-generated errors (themselves not very useful for the client to consume) are returned and suppressed when they occur. Configuring the existing and future Zod schemas into using consistent and human readable error messages would improve error management across the entire span of the application.