redux-persist: Stop erroring on a case we've seen empirically

[chrisbobbe]

As discussed in the office today.

[gnprice]

Thanks for spotting this!

Fixed the lint error and added a few more words to the comment, and merged.

---

Also FTR: I said in the office that this invariant was making this error into a crash, but looking closer at the code now I see that isn't the case -- it just causes this one async function to fail and the resulting Promise to reject. So the symptoms should be identical to what they were for the same inconsistent-storage condition before we made the refactoring that added this invariant, namely that there won't be data for the given key and that subtree of the state will remain at its initial value. Which in turn promptly gets clobbered by the initial fetch, papering over the issue, as described at #4841.

[chrisbobbe]

Great, thanks for the review and fixes!