REST API Error Handling Guide

Design a comprehensive REST API error handling system for [FRAMEWORK: Express / Fastify / Django / Rails / Spring].

Provide:
1. **Error Response Format**: Consistent JSON structure for all errors

2. **HTTP Status Code Guide**: When to use each code (400, 401, 403, 404, 409, 422, 429, 500, 503)
3. **Custom Error Classes**: Hierarchy of error types with codes
4. **Validation Errors**: How to format field-level validation errors
5. **Global Error Middleware**: Catch-all handler implementation
6. **Logging Integration**: What to log for each error type
7. **Rate Limiting Errors**: Headers and response format
8. **Authentication Errors**: Token expired, invalid, missing — different responses
9. **Client-Friendly Messages**: User-facing vs developer-facing error messages
10. **Complete Code**: Full implementation with examples

Design a comprehensive REST API error handling system for [FRAMEWORK: Express / Fastify / Django / Rails / Spring].

Provide:
1. **Error Response Format**: Consistent JSON structure for all errors
2. **HTTP Status Code Guide**: When to use each code (400, 401, 403, 404, 409, 422, 429, 500, 503)
3. **Custom Error Classes**: Hierarchy of error types with codes
4. **Validation Errors**: How to format field-level validation errors
5. **Global Error Middleware**: Catch-all handler implementation
6. **Logging Integration**: What to log for each error type
7. **Rate Limiting Errors**: Headers and response format
8. **Authentication Errors**: Token expired, invalid, missing — different responses
9. **Client-Friendly Messages**: User-facing vs developer-facing error messages
10. **Complete Code**: Full implementation with examples