Fix README error-handling example: AgentScoreError has no .message attr (#34)

## Summary
- The README's error-handling example printed `e.message`, but
`AgentScoreError` exposes its text via `str(e)`, not a `.message`
attribute (`hasattr(err, "message")` is `False`).
- Swap `print(e.code, e.message)` → `print(e.code, str(e))` so the
snippet runs as written.

Docs-only; no version bump.

## Test plan
- [x] `str(e)` returns the error text; `.message` is not an attribute on
`AgentScoreError`
- [x] pre-push hooks (ty, vulture) green

Author: vvillait88

README.md

@@ -211,7 +211,7 @@ except QuotaExceededError as e:
 except AgentScoreTimeoutError:
     print("Network timeout — retry with backoff.")
 except AgentScoreError as e:
-    print(e.code, e.message)
+    print(e.code, str(e))
 ```
 
 ## Quota observability