Rewrite errors observable

[fpillet]

This PR rewrites both the creation of Action's error observable inside the Action itself and restructures error handling in the execute function.

The motivation is to try and fix #63. The only plausible reason that could explain this issue is that the notEnabledError observable previously created was derived from inputs. Since executionObservable (the root observable for all other work inside action) was derived from inputs too, one couldn't guarantee the order of processing on input update, depending on who subscribed to what and in what order.

To fix the issue, I decided to have a single point of contact with inputs, and derive everything from it. This led to some not-perfectly-Rx-y code where I push errors to an internal subject using side effects, but it really is less important than getting Action to perform right in all circumstances.

I still have to find a way to reproduce issue #63, waiting on @tomburns to confirm whether this change fixes his issues.

[fpillet]

Ok so @tomburns confirmed that the fix indeed resolves #63. Looks like we're good to go with this one.

[tomburns]

Regarding a repro and possible test case for this:

In our case, we were doing the following, all in the same method:

1. instantiating the Action
2. subscribing to elements
3. subscribing to errors
4. passing the Action off to its consumer.

[ishkawa]

Is throw ActionError.underlyingError($0) necessary?
The error seems to appear in errors subject too.

If that is true, catchError { _ in Observable.empty() } is more suitable.

[fpillet]

If you look at the previous code, it was acting exactly like my code -- propagate errors to the execution observable returned by the execute() function, both for errors produced by the execution, and for the notEnabled error. I kept the same behaviour.

[ishkawa]

I got it 👍

[ishkawa]

nit: Introducing variable for the execution observable makes this part more readable.

let execution = workFactory(input)
    .do(onError: { errorsSubject.onNext(.underlyingError($0)) })
    .shareReplay(1)

return Observable.of(execution)

[fpillet]

I don't see how this makes the code more readable. Actually I think it makes it less readable (but that's nitpicking too)

[ishkawa]

OK. Let's leave it as it is.

[ashfurrow]

I'll try to get this released tomorrow, thanks again 👍

[ashfurrow]

Okay, all released for CocoaPods and Carthage!

[fpillet]

Thanks, Ash! 👏