Support custom error messages when validation fails on minimal APIs

This issue captures user feedback from https://twitter.com/davidfowl/status/1428228518953984005?s=20.

Currently, the feature to support treating parameters that are nullable or have default values as optional will return a status code and no error message as part of the response. This is largely because (1) it builds on top of the existing try-parse parsing logic which does nothing and (2) when we were working through the feature we had punted on the problem of what message to send back.

Users want to be be able to customize the message to:
- Provide clearer info to the client
- Conform to a specific API implementation