routing: payment splitting pre-check

This commit reverts cb4cd49dc8 to bring
back the insufficient local balance failure.

Distinguishing betweeen this failure and a regular "no route" failure
prevents meaningless htlcs from being sent out.

channeldb/payments.go

@@ -116,9 +116,6 @@ const (
 
 	// FailureReasonInsufficientBalance indicates that we didn't have enough
 	// balance to complete the payment.
-	//
-	// This reason isn't assigned anymore, but may still exist for older
-	// payments.
 	FailureReasonInsufficientBalance FailureReason = 4
 
 	// TODO(halseth): cancel state.