Use better asserts in voting lib

[pscott]

Supersedes #90 and #206
Closes #85

[Orland0x]

looks good, found one more one:
https://github.com/snapshot-labs/sx-core/blob/1551fbd2e2ee44220aa61af1a79bfcdd64bedc82/contracts/starknet/lib/voting.cairo#L440

If proposal doesnt exist then the vote will fail with "Voting has not started yet" so we should add an assert not zero