`SetWatchErrorHandler` claims it will fail if Run() has already started.
But if they are called concurrently, it will actually trigger a data
race.
With this PR:
```
62702 runs so far, 0 failures (100.00% pass rate). 59.152682ms avg, 189.068387ms max, 26.623785ms min
```
Without this PR:
```
5012 runs so far, 38 failures (99.25% pass rate). 58.675502ms avg, 186.018084ms max, 29.468104ms min
```
Kubernetes-commit: 35d2431b3a89c5bd693846952e9d27ce4e3a0754