htlcswitch: add new LinkFailureAction enum

In this commit, we add a new LinkFailureAction enum to take over the old
force close bool. Force closing isn't the only thing we might want to do
when we decide to fail the link, so this is a prep refactoring for an
upcoming change.

htlcswitch/linkfailure.go

@@ -53,6 +53,19 @@ const (
 	ErrCircuitError
 )
 
+// LinkFailureAction is an enum-like type that describes the action that should
+// be taken in response to a link failure.
+type LinkFailureAction uint8
+
+const (
+	// LinkFailureForceNone indicates no action is to be taken.
+	LinkFailureForceNone LinkFailureAction = iota
+
+	// LinkFailureForceClose indicates that the channel should be force
+	// closed.
+	LinkFailureForceClose
+)
+
 // LinkFailureError encapsulates an error that will make us fail the current
 // link. It contains the necessary information needed to determine if we should
 // force close the channel in the process, and if any error data should be sent
@@ -61,9 +74,8 @@ type LinkFailureError struct {
 	// code is the type of error this LinkFailureError encapsulates.
 	code errorCode
 
-	// ForceClose indicates whether we should force close the channel
-	// because of this error.
-	ForceClose bool
+	// FailureAction describes what we should do to fail the channel.
+	FailureAction LinkFailureAction
 
 	// PermanentFailure indicates whether this failure is permanent, and
 	// the channel should not be attempted loaded again.