Merge #4350

4350: [BFT] Reporting compliance engine protocol violations  r=durkmurder a=durkmurder

### Context

This is a follow-up PR which addresses outstanding comments in [previous](#4174) PR.
In compliance engine I have removed usage of `engine.InvalidInputError` since I wanted to consolidate slashing evidence reporting in one place(which works with `model.InvalidProposalError`) and `engine.InvalidInputError` was just adding another level of wrapping without real benefit. I've decided to replace it with `model.InvalidProposalError` which simplified error handling. 

Reference comments: [1](#4174 (comment)), [2](#4174 (comment)), [3](#4174 (comment))

Co-authored-by: Yurii Oleksyshyn <yuraolex@gmail.com>
Co-authored-by: Alexander Hentschel <alex.hentschel@axiomzen.co>

engine/collection/compliance/core.go

@@ -396,7 +396,8 @@ func checkForAndLogOutdatedInputError(err error, log zerolog.Logger) bool {
 func checkForAndLogUnverifiableInputError(err error, log zerolog.Logger) bool {
 	if engine.IsUnverifiableInputError(err) {
 		// the block cannot be validated
-		log.Err(err).Msg("received unverifiable block proposal; this is an indicator of a proposal that cannot be verified under current state")
+		log.Warn().Err(err).Msg("received collection proposal with unknown reference block; " +
+			"this might be an indicator that the node is slightly behind or the proposer published an invalid collection")
 		return true
 	}
 	return false

engine/common/follower/cache/cache.go

@@ -49,7 +49,7 @@ type Cache struct {
 	byView   map[uint64]BlocksByID          // lookup of blocks by their respective view; used to detect equivocation
 	byParent map[flow.Identifier]BlocksByID // lookup of blocks by their parentID, for finding a block's known children
 
-	notifier   hotstuff.ProposalViolationConsumer // equivocation will be reported using this notifier
+	notifier   hotstuff.ProposalViolationConsumer // equivocations will be reported using this notifier
 	lowestView counters.StrictMonotonousCounter   // lowest view that the cache accepts blocks for
 }
 

engine/consensus/compliance/core.go

@@ -340,9 +340,9 @@ func (c *Core) processBlockProposal(proposal *flow.Block) error {
 			// We know:
 			//  - the parent of this block is valid and was appended to the state (ie. we knew the epoch for it)
 			//  - if we then see this for the child, one of two things must have happened:
-			//    1. the proposer malicious created the block for a view very far in the future (it's invalid)
+			//    1. the proposer maliciously created the block for a view very far in the future (it's invalid)
 			//      -> in this case we can disregard the block
-			//    2. no blocks have been finalized within  the epoch commitment deadline, and the epoch ended
+			//    2. no blocks have been finalized within the epoch commitment deadline, and the epoch ended
 			//       (breaking a critical assumption - see EpochCommitSafetyThreshold in protocol.Params for details)
 			//      -> in this case, the network has encountered a critical failure
 			//  - we assume in general that Case 2 will not happen, therefore this must be Case 1 - an invalid block
@@ -428,7 +428,8 @@ func checkForAndLogOutdatedInputError(err error, log zerolog.Logger) bool {
 func checkForAndLogUnverifiableInputError(err error, log zerolog.Logger) bool {
 	if engine.IsUnverifiableInputError(err) {
 		// the block cannot be validated
-		log.Err(err).Msg("received unverifiable block proposal; this is an indicator of a proposal that cannot be verified under current state")
+		log.Warn().Err(err).Msg("received unverifiable block proposal; " +
+			"this might be an indicator that a malicious proposer is generating detached blocks very far ahead")
 		return true
 	}
 	return false

state/cluster/badger/mutator.go

@@ -181,7 +181,7 @@ func (m *MutableState) checkHeaderValidity(candidate *cluster.Block) error {
 // checkConnectsToFinalizedState validates that the candidate block connects to
 // the latest finalized state (ie. is not extending an orphaned fork).
 // Expected error returns:
-//   - state.UnverifiableExtensionError if the candidate extends an orphaned fork
+//   - state.OutdatedExtensionError if the candidate extends an orphaned fork
 func (m *MutableState) checkConnectsToFinalizedState(ctx extendContext) error {
 	header := ctx.candidate.Header
 	finalizedID := ctx.finalizedClusterBlock.ID()