Return consistent error responses

Use a consistent error response format across all endpoints:

```json
{
"error": {
"code": "validation_error",
"message": "Validation failed",
"details": [
{ "field": "email", "message": "is invalid" },
{ "field": "name", "message": "can't be blank" }
]
}
}
```

Include:
- Machine-readable error code
- Human-readable message
- Field-level details for validation errors