feat: kadabra, the app auto-updater (!268)

coop-cloud/organising#236

Autoupdater `kadabra` is ready for testing.
It should run on the server, check for available minor/patch updates and automatically upgrade the apps.

Co-authored-by: Moritz <moritz.m@local-it.org>
Reviewed-on: coop-cloud/abra#268

pkg/recipe/recipe.go

@@ -310,8 +310,7 @@ func EnsureVersion(recipeName, version string) error {
 	logrus.Debugf("read %s as tags for recipe %s", strings.Join(parsedTags, ", "), recipeName)
 
 	if tagRef.String() == "" {
-		logrus.Warnf("no published release discovered for %s", recipeName)
-		return nil
+		return fmt.Errorf("no published release discovered for %s", recipeName)
 	}
 
 	worktree, err := repo.Worktree()