Add check for app update (#88)

* add check for app update; fix ETUI error handling

* validate user args

imgbom/event/event.go

@@ -3,6 +3,7 @@ package event
 import "github.com/wagoodman/go-partybus"
 
 const (
-	CatalogerStarted  partybus.EventType = "cataloger-started-event"
-	CatalogerFinished partybus.EventType = "cataloger-finished-event"
+	AppUpdateAvailable partybus.EventType = "app-update-available"
+	CatalogerStarted   partybus.EventType = "cataloger-started-event"
+	CatalogerFinished  partybus.EventType = "cataloger-finished-event"
 )

imgbom/event/parsers/parsers.go

@@ -59,3 +59,16 @@ func ParseCatalogerFinished(e partybus.Event) (presenter.Presenter, error) {
 
 	return pres, nil
 }
+
+func ParseAppUpdateAvailable(e partybus.Event) (string, error) {
+	if err := checkEventType(e.Type, event.AppUpdateAvailable); err != nil {
+		return "", err
+	}
+
+	newVersion, ok := e.Value.(string)
+	if !ok {
+		return "", newPayloadErr(e.Type, "Value", e.Value)
+	}
+
+	return newVersion, nil
+}