use latest solidity features: error-messages, returning arrays

Feature Description

solidity is getting safer, more informative & powerful:

Motivation and Context

https://github.com/ethereum/solidity/releases/tag/v0.4.22

Possible Implementation

• informative: meaningful error messages in case of failures
• refactor: maybe returning arrays is indeed helpful somewhere, e.g. in computing the score and payoffs.

External Resources and Context

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