Improve handling of problematic ballots

[carlschroedl]

As a user of the RankedPairsCalculator I want clear feedback about problems from incomplete or invalid ballots (rather than silent failures or cryptic error messages) so that I can quickly diagnose and fix problematic ballots.

An incomplete ballot is one which does not rank all candidates in the election This could be anything from the omission of all candidates tied for last, or the omission of a candidate from the middle of the order ex. 1,2,3,5,6.

An invalid ballot is one which has multiple candidates with the same id, or candidates which are not in this election.

This issue may involve adding a "problemBallots" field to a Results object. Alternatively it could involve throwing a custom Exception. This issue may involve adding a 'force' boolean which would allow a user to compute the election result without considering the problematic ballots.