Improve collector configuration preview errors

Preview errors in the collector configuration create and edit pages may not be formatted and are sometimes shown on a single line:

## Expected Behavior

We should try to improve the formatting of those errors, force line breaks using CSS, and/or use a wide modal to make those error messages easier to read.

## Current Behavior

Preview errors are not formatted and are sometimes displayed in a single line, making it hard to read that information while writing a configuration.

## Steps to Reproduce (for bugs)
1. Go to the Sidecar -> Configuration -> New/Edit Configuration page
2. Write a configuration with an error (i.e. use an non-existing variable)
3. Click on preview and see the error

## Your Environment
* Graylog Version: Graylog 3.0.0-beta.2-SNAPSHOT