lnwallet: during channel resynchronization detect lack of commitment window

In this commit, we extend the ProcessChanSyncMsg to detect a case where
we don’t have the necessary revocation window to send out a new commit.
This can arise if the remote party sends us a new state, but we haven’t
yet fully processed their FundingLocked message yet, so we would be
unable to create a new commitment state.

We fix this by enumerating each of our actions in the case of an error.
If we get ErrNoWindow, then this indicates that we can’t give the
remote party the commitment we would like to optimistically send over.
This isn’t an issue though, as in the next round, we’ll resynchronize
our state.

lnwallet/channel.go

@@ -3161,16 +3161,30 @@ func (lc *LightningChannel) ProcessChanSyncMsg(msg *lnwire.ChannelReestablish) (
 			lc.remoteCommitChain.tip().height {
 
 			commitSig, htlcSigs, err := lc.SignNextCommitment()
-			if err != nil {
+			switch {
+
+			// If we signed this state, then we'll accumulate
+			// another update to send over.
+			case err == nil:
+				updates = append(updates, &lnwire.CommitSig{
+					ChanID: lnwire.NewChanIDFromOutPoint(
+						&lc.channelState.FundingOutpoint,
+					),
+					CommitSig: commitSig,
+					HtlcSigs:  htlcSigs,
+				})
+
+			// If we get a failure due to not knowing their next
+			// point, then this is fine as they'll either send
+			// FundingLocked, or revoke their next state to allow
+			// us to continue forwards.
+			case err == ErrNoWindow:
+
+			// Otherwise, this is an error and we'll treat it as
+			// such.
+			default:
 				return nil, err
 			}
-			updates = append(updates, &lnwire.CommitSig{
-				ChanID: lnwire.NewChanIDFromOutPoint(
-					&lc.channelState.FundingOutpoint,
-				),
-				CommitSig: commitSig,
-				HtlcSigs:  htlcSigs,
-			})
 		}
 
 	// If we don't owe the remote party a revocation, but their value for