update test error string

Co-authored-by: Anthonios Partheniou <partheniou@google.com>

Author: daniel-sanche

tests/unit/test_retry.py

@@ -172,7 +172,7 @@ def inner_gen():
 
         return inner_gen()
 
-    with pytest.raises(ValueError, match="Sleep generator"):
+    with pytest.raises(ValueError, match="Sleep generator stopped yielding sleep values"):
         gen = RetryableGenerator(target_fn, lambda x: True, [], None)
         next(gen)