Return an error on unknown commands [#15]

I'm probably overthinking this--explicitly calling help is a strange and
unusual case--but it doesn't really hurt, so I'm going for it.

internal/run/help.go

@@ -10,8 +10,15 @@ type Help struct {
 }
 
 // Execute executes the `helm help` command.
-func (h *Help) Execute(_ Config) error {
-	return h.cmd.Run()
+func (h *Help) Execute(cfg Config) error {
+	if err := h.cmd.Run(); err != nil {
+		return fmt.Errorf("while running '%s': %w", h.cmd.String(), err)
+	}
+
+	if cfg.HelmCommand == "help" {
+		return nil
+	}
+	return fmt.Errorf("unknown command '%s'", cfg.HelmCommand)
 }
 
 // Prepare gets the Help ready to execute.