Data Adapter which failed during `doStart()` is still usable.

## Expected Behavior
A data adapter which failed during `doStart()` is not accessible using the frontend and/or the API.

## Current Behavior
A data adapter which failed during `doStart()` is still accessible using its `doGet()` method. This leads to using a partially/not initialized data adapter, which fails during the method.

## Possible Solution

## Steps to Reproduce (for bugs)

1. Create a HTTPJsonPath data adapter, give it an indefinite single value path
2. Navigate to this data adapters detail page
3. Use an arbitrary key for lookup
4. Check the server logs

## Context

## Your Environment

* Graylog Version: 3.0.0-SNAPSHOT+9cfcf41
* Elasticsearch Version:
* MongoDB Version:
* Operating System:
* Browser version: