[ISSUE #4795] Replace this exception in beginFlush() with log print and return false (#4797)

Author: jevinjiang

Diff for: eventmesh-openconnect/eventmesh-openconnect-offsetmgmt-plugin/eventmesh-openconnect-offsetmgmt-api/src/main/java/org/apache/eventmesh/openconnect/offsetmgmt/api/storage/OffsetStorageWriterImpl.java

@@ -84,7 +84,8 @@ private boolean isFlushing() {
      */
     public synchronized boolean beginFlush() {
         if (isFlushing()) {
-            throw new RuntimeException("OffsetStorageWriter is already flushing");
+            log.warn("OffsetStorageWriter is already flushing");
+            return false;
         }
         if (data.isEmpty()) {
             return false;