Graylog tries to create deflector aliases as indexes on restart / or faulty log message

## Current Behavior
After each restart of elastic we get messages similar to this one in the graylog server.log

2018-12-03T13:41:11.195+01:00 WARN [Messages] Failed to index message: index= id= error=<{“type”:“invalid_index_name_exception”,“reason”:“Invalid index name [del3month_deflector], already exists as alias”,“index_uuid”:“na”,“index”:“del3month_deflector”}>

It’s like graylog tries to create the deflector aliases as indexes but fails due to the fact that they already exists as aliases.

I posted a question in the forum (https://community.graylog.org/t/graylog-tries-to-create-deflector-aliases-as-indexes-on-restart/7876) and was asked to file a bug so here I am.

## Expected Behavior
A log message describing what the problem actually is in case it's a faulty log message.

## Steps to Reproduce (for bugs)
1. In my system all I need to do to recreate the bug is to restart elasticsearch.

## Your Environment

* Single node system
* Graylog Version: 2.4.6+ceaa7e4
* Elasticsearch Version: 5.6.11
* Operating System: CentOS 7 (Linux 3.10.0-862.11.6.el7.x86_64)
* JVM: Oracle Corporation 1.8.0_181 on

Thanks for your help and let me know if you need any additional information.