Remove References to Sanity in Combine Reducers

[seanmhanson]

The file combineReducers.js contains language like assertReducerSanity and sanityError that is generally viewed as ableist and potentially offensive to people living with mental illness. In addition, it doesn't accurately describe what we're trying to do in this check, and using an idom like this also makes it less approachable for developers whose first language is not English.

I'll throw a PR up in a second since this is a super tiny change, but I'm proposing to call it assertReducerShape and describe the error as a shapeAssertionError so that we're more precise and more inclusive.

[timdorr]

Closing in favor of the PR you opened up. (Thanks for that!)