* MM-18167: fix KV* helpers error handling
Returning a `nil` `*model.AppError` does not make a `nil` `error`
interface. As a consequence, all of the KV* helper methods would always
appear to fail, even if the underlying API call was successful.
Fix this mismatch by explicitly assigning `appErr` in the helpers and
only ever returning a `nil` `error` interface. Extend unit tests to
achieve 100% coverage of the associated files.
In the long run, we must change all function signatures to return the
`error` interface instead of the abomination that is returning
`*model.AppError` today.
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