Preparation errors should no longer be a InvalidCandidate variant

[eskimor]

Since we have prechecking enabled, we no longer want to treat preparation error as if the candidate was invalid. Hence PrepareError should not be a variant of the InvalidCandidate enum, but instead be consolidated with `ValidationError::InternalError.

In particular InternalError represents spurious errors, while PrepareError represents deterministic errors. This differentiation is still useful, so instead of making those two errors one, I would suggest to only move PrepareError to the ValidationError enum, but reconsider the naming as both InternalError and PrepareError are prepare errors.