Need for more human-readable/understandable log entries.

## What?

This comes from z-4672. As a Graylog operator, I would like to be able to more easily discern the cause of an error in the logs when encountering messages like:

```

2021-02-18T10:17:42.311+01:00 ERROR [BlockingBatchedESOutput] Unable to flush message buffer
java.lang.ClassCastException: Cannot cast java.lang.Integer to java.lang.String
at java.lang.Class.cast(Class.java:3369) ~[?:1.8.0_275]
at org.graylog2.plugin.Message.getFieldAs(Message.java:643) ~[graylog.jar:?]
at org.graylog2.plugin.Message.getSource(Message.java:469) ~[graylog.jar:?]
at org.graylog2.plugin.Message.toElasticSearchObject(Message.java:391) ~[graylog.jar:?]
at org.graylog2.indexer.messages.Messages.bulkIndexChunk(Messages.java:268) ~[graylog.jar:?]
at org.graylog2.indexer.messages.Messages.bulkIndexChunked(Messages.java:226) ~[graylog.jar:?]
at org.graylog2.indexer.messages.Messages.bulkIndex(Messages.java:188) ~[graylog.jar:?]
at org.graylog2.indexer.messages.Messages.bulkIndex(Messages.java:175) ~[graylog.jar:?]
at org.graylog2.outputs.ElasticSearchOutput.writeMessageEntries(ElasticSearchOutput.java:111) ~[graylog.jar:?]
at org.graylog2.outputs.BlockingBatchedESOutput.flush(BlockingBatchedESOutput.java:129) [graylog.jar:?]
at org.graylog2.outputs.BlockingBatchedESOutput.forceFlushIfTimedout(BlockingBatchedESOutput.java:154) [graylog.jar:?]
at org.graylog2.periodical.BatchedElasticSearchOutputFlushThread.doRun(BatchedElasticSearchOutputFlushThread.java:82) [graylog.jar:?]
at org.graylog2.plugin.periodical.Periodical.run(Periodical.java:77) [graylog.jar:?]
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511) [?:1.8.0_275]
at java.util.concurrent.FutureTask.runAndReset(FutureTask.java:308) [?:1.8.0_275]
at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.access$301(ScheduledThreadPoolExecutor.java:180) [?:1.8.0_275]
at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:294) [?:1.8.0_275]
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149) [?:1.8.0_275]
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) [?:1.8.0_275]
at java.lang.Thread.run(Thread.java:748) [?:1.8.0_275]
```

Or

```
2021-02-18T15:42:40.180+01:00 WARN [DefaultExceptionMonitor] Unexpected exception.
java.nio.channels.ClosedSelectorException: null
at sun.nio.ch.SelectorImpl.lockAndDoSelect(SelectorImpl.java:83) ~[?:1.8.0_275]
at sun.nio.ch.SelectorImpl.select(SelectorImpl.java:97) ~[?:1.8.0_275]
at org.apache.mina.transport.socket.nio.NioProcessor.select(NioProcessor.java:112) ~[graylog.jar:?]
at org.apache.mina.core.polling.AbstractPollingIoProcessor$Processor.run(AbstractPollingIoProcessor.java:616) [graylog.jar:?]
at org.apache.mina.util.NamePreservingRunnable.run(NamePreservingRunnable.java:64) [graylog.jar:?]
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149) [?:1.8.0_275]
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) [?:1.8.0_275]
at java.lang.Thread.run(Thread.java:748) [?:1.8.0_275]
```

## Why?

This doesn't tell me as an operator:

1. Anything useful or descriptive about what caused the error
2. If I should be worried or not
3. Any sort of hind about what I can do to solve the issue.