Harden '_list_entries' further against backoff failure. (#5551)

Closes #5303.

Author: tseaver

tests/system/test_system.py

@@ -65,7 +65,8 @@ def _list_entries(logger):
     :returns: List of all entries consumed.
     """
     inner = RetryResult(_has_entries)(_consume_entries)
-    outer = RetryErrors((ServiceUnavailable, ResourceExhausted))(inner)
+    outer = RetryErrors(
+        (ServiceUnavailable, ResourceExhausted), max_tries=9)(inner)
     return outer(logger)