Expand custom error messages to include API specific errors

[juan88]

Is it possible to add or enlarge an existing example so as to include custom error messages that should be returned to the consumer of the API we build?

I'm not able to add a custom validator and add as well a application specific defined message that is more descriptive than:

{"error":"Key: 'Booking.CheckOut' Error:Field validation for 'CheckOut' failed on the 'required' tag"}%

Thanks!