Make it possible to log when an error occurs
We are creating a redis rate limiter, with an in-memory insurance limiter. We would like to be notified whenever a call to redis fails, so we can do something about it :)

The way I've done it for now is to subclass `RateLimiterRedis`:

```ts
class LenusRateLimiterRedis extends RateLimiterRedis {
  _handleError(err, funcName, resolve, reject, key) {
    errorTracker.capture('Error in redis rate limiter', { key, funcName }, err)
    super._handleError(err, funcName, resolve, reject, key)
  }
}
```

This works fine, but since `_handleError` is not part of the ts interface definition we won't be notified if the definition of `_handleError` changes.

My idea would be to either:
* Make `_handleError` (or a similar method) part of the public interface, so we can subclass is properly
* Add an `onError` callback to options
* Emit an `error` event

Whichever option you like most, I would be happy to implement it :)