Automatic merge from submit-queue (batch tested with PRs 61871, 61890, 61786). If you want to cherry-pick this change to another branch, please follow the instructions <a href="https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md">here</a>.

Add unit testcases for ensureExternalLoadBalancer to make sure it doesn't panic when errors raised.

**What this PR does / why we need it**:
Add unit testcases for ensureExternalLoadBalancer to make sure it doesn't panic when errors raised. Increase code coverage from 76.5% to 81.5%.

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**Release note**: 

```release-note
NONE
```

\assign @MrHohn