Fix proposal block number error

[ethedev]

Fix proposal block number error

Error shows if you set an invalid block number, and the block number must be bigger than the block where the token has been created at, this should not console log error but show the user more info what happened in there.

Snapshot proposal link:
https://snapshot.page/#/nft/proposal/QmPGuJzBuuajYMHEyTEomnTbyd46wECvh7QHYkpKTcEVik
IPFS link:
https://ipfs.io/ipfs/QmPGuJzBuuajYMHEyTEomnTbyd46wECvh7QHYkpKTcEVik

Error log

TypeError: Cannot read property 'proposal' of undefined
    at Proposal.vue:204
    at u (runtime.js:45)
    at Generator._invoke (runtime.js:274)
    at Generator.forEach.e.<computed> [as next] (runtime.js:97)
    at n (asyncToGenerator.js:3)
    at s (asyncToGenerator.js:25)