[improve][broker] Add an error log to troubleshoot the failure of sta…

…rting broker registry. (#22065)

(cherry picked from commit baddda5)

pulsar-broker/src/main/java/org/apache/pulsar/broker/loadbalance/extensions/ExtensibleLoadManagerImpl.java

@@ -390,6 +390,8 @@ public void start() throws PulsarServerException {
             this.initWaiter.countDown();
             this.started = true;
         } catch (Exception ex) {
+            log.error("Failed to start the extensible load balance and close broker registry {}.",
+                    this.brokerRegistry, ex);
             if (this.brokerRegistry != null) {
                 brokerRegistry.close();
             }