consumer: return descriptive error when a nil state is passed into NewJSONFileStore

[kanwals]

Return a more descriptive error if a nil state is passed into NewJSONFileStore.

Didn't add/modify tests as the change is pretty trivial. LMK if you think otherwise.

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This change is 

[jgraettinger]

Thanks. This is actually a case where i'd mostly focus on testing, because a defect here can silently persist bad data that's only discovered much later, as you saw (aka, potential for a big production headache). Please also update godocs since we're putting a new constraint on callers.

Reviewable status: 0 of 1 files reviewed, all discussions resolved

[kanwals]

Just a headsup that I'm aware of the response, and would likely need some time (probably EOW) to get to this.

[kanwals]

Took a stab at it. I wasn't completely sure of what the right location of the test should be, and LMK if this doesn't look good.

PTAL @jgraettinger

Reviewable status: 0 of 4 files reviewed, all discussions resolved

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consumer/shard.go, line 248 at r3 (raw file):

	s.wg.Wait()

	if s.store != nil && !reflect.ValueOf(s.store).IsNil() {

This one was a gotcha, where control would still go inside the if since s.store was a typed nil.