Fix README error-handling example: AgentScoreError has no .message attr

The except block printed e.message, but AgentScoreError exposes its text via
str(e), not a .message attribute. Use str(e) so the example runs.

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