Add contextual error code for flux diff kustomization

If implemented, calling the diff command on kustomization will return 0,
1(if changes are identified), >1 for errors.

Signed-off-by: Soule BA <soule@weave.works>

cmd/flux/diff.go

@@ -23,7 +23,7 @@ import (
 var diffCmd = &cobra.Command{
 	Use:   "diff",
 	Short: "Diff a flux resource",
-	Long:  "The diff command is used to do a server-side dry-run on flux resources, then output the diff.",
+	Long:  "The diff command is used to do a server-side dry-run on flux resources, then prints the diff.",
 }
 
 func init() {