fix: handle case where deploying app image does not exist

This happens if ps:scale is called before a deploy.

Also refactor GetDeployingAppImageName() to return an error as the second return parameter.

plugins/app-json/appjson.go

@@ -138,7 +138,10 @@ func getDokkuAppShell(appName string) string {
 }
 
 func executeScript(appName string, imageTag string, phase string) error {
-	image := common.GetDeployingAppImageName(appName, imageTag, "")
+	image, err := common.GetDeployingAppImageName(appName, imageTag, "")
+	if err != nil {
+		common.LogFail(err.Error())
+	}
 	command := ""
 	phaseSource := ""
 	if phase == "release" {