Throw new error when initial state is undefined

[dannielss]

Add a new error when initial state is undefined on createSlice.ts.

Issue: #2459

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[dannielss]

@markerikson it should be throw new error or console.error?

[markerikson]

Heh, sorry - I had assumed "throw", Lenz said "print a console error", so we'll go with Lenz's opinion here :)

[markerikson]

@dannielss : yeah, let's go with:

console.error('You must provide an `initialState` value that is not `undefined`.  You may have misspelled `initialState`')

[dannielss]

Updated to console.error

[markerikson]

Thanks! At a minimum this will end up going out in 1.9. If we end up doing a 1.8.x minor release before then, it'll be part of that first.