[#260] Throw better errors in `proposal_check`, add more tests

[rinn7e]

Description

Problem: Both registryDAO and treasuryDAO have a proposal_check lambda that is meant to check some conditions to approve/discard submitted proposals. Tests on such checks however are a bit lacking and don't cover all the cases of possible failure and corner cases (including possible lack of failures).

Solution: Update proposal_check to throw clearer error messages, add more tests to handle all possible cases.

Related issue(s)

Resolves #260

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