fix: report proxy shutdown failures

scripts/verifyProxy.py

@@ -226,8 +226,12 @@ def main() -> None:
             assert backend.stdin is not None
             backend.stdin.write('"stop"\n')
             backend.stdin.flush()
-            if backend.wait(timeout=5) != 0:
-                raise AssertionError("native server did not shut down cleanly behind Nginx")
+            exit_code = backend.wait(timeout=5)
+            if exit_code != 0:
+                details = backend.stderr.read().strip() if backend.stderr is not None else ""
+                raise AssertionError(
+                    f"native server did not shut down cleanly behind Nginx: exit={exit_code} {details}"
+                )
             print("PASS native graceful shutdown behind proxy", flush=True)
         finally:
             if nginx is not None and nginx.poll() is None: