Full nodes should not accept invalid blocks

[cmwaters]

Summary

Blocks can be invalid for several reasons:

• The square was incorrectly built based on the set of txs
• PFBs and NonPFBs were not correctly separated
• Signatures were invalid
• Insufficient fees or gas

Note, that because we don't actually execute the block, it's okay that some transactions aren't valid.

If a group of byzantine validators (>2/3 in voting power) commit to an invalid block, whereby an honest node rejected it in ProcessProposal, all honest nodes will still continue to execute and propagate that invalid block.

What we should do is generate BEFPs (and any other proof), propagate them around the network using go-fraud and shut down the consensus reactor so we no longer vote on any more block until social consensus has re-established the canonical path forward (perhaps in the future we can resolve some invalid blocks automatically)

Proposal

Most likely we should cache the hashes of the invalid blocks in ProcessProposal and if we seem them committed in BeginBlock we return an error to consensus which deletes the last block and no longer processes any more transactions

For Admin Use

• Not duplicate issue
• Appropriate labels applied
• Appropriate contributors tagged
• Contributor assigned/self-assigned