execute_key_op_with_retry: return RetryExhausted on budget exhaustion

[mhatrevi]

execute_key_op_with_retry in resiliency.rs (line 637) has two bugs that cause intermittent stress test failures under high contention.

Raw retryable error leaks instead of RetryExhausted
Location: resiliency.rs, Phase 2 match arms (around line 669)