Add design document for Issue #100: Enhanced error handling

This design document provides a comprehensive specification for enhancing
the generated client's error handling capabilities including:

- Rich exception hierarchy (APIError, ClientError, ServerError)
- Parsed error models from OpenAPI specifications
- Dual API pattern (simple + detailed response wrappers)
- Full HTTP context (method, URL, headers, request IDs)
- Support for multiple success codes and wildcard patterns
- Type-safe error handling without Union type overhead
- Backwards compatibility with existing HTTPException

The design includes:
- Complete architecture and component specifications
- Usage examples for all API patterns
- 10-phase implementation plan
- Design rationale and backwards compatibility analysis

Addresses Issue #100

Author: Doug Borg