edit Extractor not possible if ES did not return messages

## Expected Behavior
When in the Extractor Overview ( `Input > Manage Extractors` ) it should be possible to click the `edit` button and receive feedback.
As expected the page should be loaded to edit the extractor or - if the load of the example message is not possible - it should display some error messages and how to solve that.

## Current Behavior
Click on the Button while ES is not able to return the needed example message gives you a white page that loads forever.
Only with the help of the developer tools you are able to identify the issue and in addition you need to understand the error message.

## Possible Solution
Should the load of the example message fail the page should give the user feedback about that and not load forever.

## Context
Without a feedback the user did not know if Graylog is just slow for some reason of if any other problem is currently present in the setup.

## Your Environment

* Graylog Version: 2.4.3