When incorrectly using converter messages do not store / no errors

## Expected Behavior

When creating an extractor and using the incorrect converter, log an error upon a failed extraction.
## Current Behavior

I created a new input and was in the process of writing the extractors for it. I specifically started working on the timestamp field and extracted it in the format of

`2016 Sep 14 18:49:20`

I didn't read the docs - and realize that this is not a correct format now, but I tried applying the flexible date converter to this field. When I did that, messages were still coming in the input but nothing was getting saved - you could do a search for that whole input after creating that extractor and no messages were logged even though it shows messages being received on the inputs page.

Once this extractor was deleted and or the converter taken off, everything started flowing again. The problem here is that there were no errors issued - it simply stopped logging messages. I checked the server log as well as indexing logs to no avail.
## Possible Solution
## Steps to Reproduce (for bugs)
1. Create new extractor on an input
2. Apply an incorrect converter (I specifically and only tested the flexible date converter)
3. Messages will still flow into the input but will not get logged
## Context
## Your Environment
- Graylog Version: 2.1.1
- Elasticsearch Version: 2.3.5
- MongoDB Version: 3.2.9
- Operating System: Ubuntu 16.04
- Browser version: