[Improvement][jobplan]修改flinksql执行图返回提示 (#679)

* 修改如果任务没有insert语句，返回提示创建失败

* update note
Co-authored-by: syyang <730122+syyang99@user.noreply.gitee.com>

dlink-core/src/main/java/com/dlink/explainer/Explainer.java

@@ -296,7 +296,7 @@ public class Explainer {
             }
             return executor.getJobPlanInfoFromDataStream(datastreamPlans);
         } else {
-            return new JobPlanInfo("");
+            throw new RuntimeException("Creating job plan fails because this job doesn't contain an insert statement.");
         }
     }