Index message even when a mandatory field is missing

It should be possible to index messages even when a mandatory field like `message` is missing.
In this case Graylog could simply inject an error message saying that the message was not complete and maybe even display this in the UI.

## Expected Behavior

Messages with missing mandatory fields should be indexed if possible

## Current Behavior

Messages are dropped

## Possible Solution

Relax GELF input and auto-complete missing fields with e.g. error messages

## Context

Big companies don't always have full control over what is send to Graylog so it should be less strict about broken messages.