feat!: throw discriminable errors indicating point of failure

[mnahkies]

Previously if request validation or response validation failed you'd just get something like a ZodError without much context as to where in the request processing failed.

This change introduced a KoaRuntimeError that wraps all exceptions thrown during request processing and annotates them with additional context via a phase property.

The intention is that you can then mount a generic error handling middleware that can distinguish between request validation errors, etc.

See integration-tests/typescript-koa/src/petstore-expanded.yaml.ts for an example.

Breaking change: errors thrown are now wrapped in KoaRuntimeError objects.