The check_enough_votes method returns an awkward error

In the documentation the `EngineError::BadSealFieldSize` is stated as "Seal field has an unexpected size". However, the method `check_enough_votes` returns an error of this type when the collected votes do not exceed the threshold.

Start by locating check_enough_votes and the definition and documentation for EngineError::BadSealFieldSize. Trace the insufficient-votes path and existing error tests; done means that path reports an error whose meaning matches the documentation, with coverage for the mismatch.

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