Fix explicit invalid and normal invalid support

[Findeton]

Currently, if the question has an explicitly invalid option, the question will be marked as invalid in the verifier, regardless of the selected options.

Fix that, make the verifier also contemplate whether the vote is implicit/explicit invalid and add tests.

Changes

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1. Fix auditing questions with explicitly invalid options.
2. Show whether the ballot is explicitly or implicitly invalid.
3. Check what happens in the verifier when an inexistent option is chosen.
4. Add tests for explicitly invalid ballots.
5. Add tests for implicitly invalid ballots (min/max options).
6. Add tests for both explicitly and implicitly invalid ballots (it should show them as explicitly invalid).
7. Add test for implicitly invalid options (selected option doesn't exist).

Tasks

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1. Fix explicit invalid and normal invalid support #19
   bug +1
2. Fix explicit invalid and normal invalid support (#19) #20
   bug +1