JIRA: https://jira.spring.io/browse/INT-4447
When we get an exception during `this.lock.unlock()`, we don't revoke
leadership.
In case of external resource (e.g. JDBC) this may cause a race condition
when the second candidate is selected as leader when connection comes back
* Catch `this.lock.unlock()` exceptions and log them under DEBUG.
This way we proceed to the `handleRevoked()` logic
**Cherry-pick to 5.0.x and 4.3.x**
15fc23c2fa