[Transform] Ignore errors from existing destination indices

[prwhelan]

Description

As a result of this change: #108394

We're seeing a handful of errors related to:

org.elasticsearch.transport.RemoteTransportException: [<>][<>][indices:admin/create]
Caused by: [.metrics-endpoint.metadata_united_default/<>] org.elasticsearch.ResourceAlreadyExistsException: index [.metrics-endpoint.metadata_united_default/<>] already exists
	at org.elasticsearch.cluster.metadata.MetadataCreateIndexService.validateIndexName(MetadataCreateIndexService.java:177)
	at org.elasticsearch.cluster.metadata.MetadataCreateIndexService.validate(MetadataCreateIndexService.java:1371)
	at org.elasticsearch.cluster.metadata.MetadataCreateIndexService.applyCreateIndexRequest(MetadataCreateIndexService.java:345)
	at org.elasticsearch.cluster.metadata.MetadataCreateIndexService$1.execute(MetadataCreateIndexService.java:300)
	at org.elasticsearch.cluster.service.MasterService$UnbatchedExecutor.execute(MasterService.java:551)
	at org.elasticsearch.cluster.service.MasterService.innerExecuteTasks(MasterService.java:1050)
	at org.elasticsearch.cluster.service.MasterService.executeTasks(MasterService.java:1015)
	at org.elasticsearch.cluster.service.MasterService.executeAndPublishBatch(MasterService.java:233)
	at org.elasticsearch.cluster.service.MasterService$BatchingTaskQueue$Processor.lambda$run$2(MasterService.java:1656)
	at org.elasticsearch.action.ActionListener.run(ActionListener.java:433)
	at org.elasticsearch.cluster.service.MasterService$BatchingTaskQueue$Processor.run(MasterService.java:1653)
	at org.elasticsearch.cluster.service.MasterService$5.lambda$doRun$0(MasterService.java:1248)
	at org.elasticsearch.action.ActionListener.run(ActionListener.java:433)
	at org.elasticsearch.cluster.service.MasterService$5.doRun(MasterService.java:1227)
	at org.elasticsearch.common.util.concurrent.ThreadContext$ContextPreservingAbstractRunnable.doRun(ThreadContext.java:984)
	at org.elasticsearch.common.util.concurrent.AbstractRunnable.run(AbstractRunnable.java:26)
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1144)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:642)
	at java.lang.Thread.run(Thread.java:1570)

That should be handled here:

elasticsearch/x-pack/plugin/transform/src/main/java/org/elasticsearch/xpack/transform/persistence/TransformIndex.java

Line 204 in 96e297b

if (e instanceof ResourceAlreadyExistsException) {

We just need to unwrap the RemoteTransportException and handle the nested ResourceAlreadyExistsException

This is only happening 26 times in the last 2 weeks, and the checkpoint is retried and succeeds, so this seems low risk

[elasticsearchmachine]

Pinging @elastic/ml-core (Team:ML)