Capture errors thrown in action creators

[PsychoLlama]

I need error reducers to intercept error events, not just those gracefully returned by failure(...). This means adding a catch statement on the effect, but as I outlined over here, I can't just swallow every error. That means conditionally re-throwing after dispatch is done.

The only way that's possible is through async generator functions.

async function* createAction(type, effect) {
  try {
    yield { type, payload: effect() }
  } catch (error) {
    yield { type, error: true, payload: error }
    if (isUnknown(error)) throw error
  }
}

[PsychoLlama]

Hmm, speed bump. I had assumed the only noticeable effect would be if you call the action directly (without a redux dispatch), but while implementing the change, I realized the return value of dispatch changes to a promise. This would implicitly require all synchronous actions to return promises. I'm not comfortable with that tradeoff.

I'm pursuing synchronous iterators in #210.