change log level from error to warn when dns resolver initialize fail…

…ed (#2856)

Descriptions of the changes in this PR:

### Motivation
When start bookie, it will throws the following error message when dns resolver initialize failed.
```
[main] ERROR org.apache.bookkeeper.client.RackawareEnsemblePlacementPolicyImpl - Failed to initialize DNS Resolver org.apache.bookkeeper.net.ScriptBasedMapping, used default subnet resolver : java.lang.RuntimeException: No network topology script is found when using script based DNS resolver.
```
It is confusing for users.

### Modification
1. change the log level from error to warn.

bookkeeper-server/src/main/java/org/apache/bookkeeper/client/RackawareEnsemblePlacementPolicyImpl.java

@@ -269,7 +269,7 @@ public RackawareEnsemblePlacementPolicyImpl initialize(ClientConfiguration conf,
                 }
             } catch (RuntimeException re) {
                 if (!conf.getEnforceMinNumRacksPerWriteQuorum()) {
-                    LOG.error("Failed to initialize DNS Resolver {}, used default subnet resolver ",
+                    LOG.warn("Failed to initialize DNS Resolver {}, used default subnet resolver ",
                             dnsResolverName, re);
                     dnsResolver = new DefaultResolver(this::getDefaultRack);
                     dnsResolver.setBookieAddressResolver(bookieAddressResolver);