Race condition in SetIndexReadOnlyAndCalculateRangeJob

A quickly consecutive execution of rotating the active write index and deleting the old index
triggers this exception:
```
2019-01-29 14:05:16,904 ERROR: org.graylog2.system.jobs.SystemJobManager - Unhandled error while running SystemJob <740cbf40-23c6-11e9-8d27-4292641f89fe> [org.graylog2.indexer.indices.jobs.SetIndexReadOnlyAndCalculateRangeJob]
java.lang.IllegalArgumentException: Cat response did not contain a JSON Array
at io.searchbox.core.Cat.parseResponseBody(Cat.java:61) ~[jest-common-2.4.15+jackson.jar:?]
at io.searchbox.action.AbstractAction.createNewElasticSearchResult(AbstractAction.java:71) ~[jest-common-2.4.15+jackson.jar:?]
at io.searchbox.core.Cat.createNewElasticSearchResult(Cat.java:44) ~[jest-common-2.4.15+jackson.jar:?]
at io.searchbox.core.Cat.createNewElasticSearchResult(Cat.java:16) ~[jest-common-2.4.15+jackson.jar:?]
at io.searchbox.client.http.JestHttpClient.deserializeResponse(JestHttpClient.java:212) ~[jest-2.4.15+jackson.jar:?]
at io.searchbox.client.http.JestHttpClient.execute(JestHttpClient.java:88) ~[jest-2.4.15+jackson.jar:?]
at org.graylog2.indexer.cluster.jest.JestUtils.execute(JestUtils.java:46) ~[classes/:?]
at org.graylog2.indexer.cluster.jest.JestUtils.execute(JestUtils.java:62) ~[classes/:?]
at org.graylog2.indexer.indices.Indices.catIndices(Indices.java:581) ~[classes/:?]
at org.graylog2.indexer.indices.Indices.getClosedIndices(Indices.java:520) ~[classes/:?]
at org.graylog2.indexer.indices.Indices.isClosed(Indices.java:566) ~[classes/:?]
at org.graylog2.indexer.indices.jobs.SetIndexReadOnlyAndCalculateRangeJob.execute(SetIndexReadOnlyAndCalculateRangeJob.java:66) ~[classes/:?]
at org.graylog2.system.jobs.SystemJobManager$1.run(SystemJobManager.java:89) [classes/:?]
at com.codahale.metrics.InstrumentedScheduledExecutorService$InstrumentedRunnable.run(InstrumentedScheduledExecutorService.java:241) [metrics-core-4.0.3.jar:4.0.3]
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511) [?:1.8.0_191]
at java.util.concurrent.FutureTask.run(FutureTask.java:266) [?:1.8.0_191]
at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.access$201(ScheduledThreadPoolExecutor.java:180) [?:1.8.0_191]
at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:293) [?:1.8.0_191]
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149) [?:1.8.0_191]
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) [?:1.8.0_191]
at java.lang.Thread.run(Thread.java:748) [?:1.8.0_191]
```
## Steps to Reproduce (for bugs)
1. Go to an index set
2. Click "Rotate active write index"
3. Shortly thereafter delete one index from the set

* Graylog Version: 3.0 beta 4
* Elasticsearch Version: 5.6