Can we make navigationStore.defaultSuccess and navigationStore.defaultFailure customizable?

### Version
Tell us which versions you are using:
- react-native-router-flux v4.0.6
- react v16.8.3
- react-native v0.59.8

### Expected behaviour

If `Component.onEnter` or `Component.onExit` fails, I want to be able to catch and log errors.

### Actual behaviour

- it works for `onEnter` right now, the only caveat is that you need to declare a `failure()` handler on each ``, making the feature unusable on the long term
- there is no way to achieve this of `onExit`

Code is located here:

https://github.com/aksonov/react-native-router-flux/blob/1a1d0f65d0db76084dbe47f90bb0eb495cf8792b/src/navigationStore.js#L530-L564