Validation Error Types should have Schemas

[crutcher]

The existing validation machinery here:
https://github.com/crutcher/loom/tree/main/tensortapestry-common/src/main/java/org/tensortapestry/common/validation

does an excellent job of collecting complex validation errors into JSON and/or to text output for exceptions.

But there's no enforcement that a given error type has particular fields set; so later error-aware tooling will be unstable.

This task is to cookup a validation schema mechanism, and tie it to error types (via a URI?)