Pipeline Source Editor: Highlight Editor when an error is present

## Expected Behavior
If the user has entered a invalid rule and is clicking on updating the rule. The source input field should be highlighted since it contains the reason the form is not valid.

## Current Behavior
After clicking on update the a toaster notification pops up containing a really unhelpful message about an error fetching something:

## Possible Solution
- Better toaster error message
- Highlighting the source field if it contains an error.

## Steps to Reproduce (for bugs)
1. Create a new rule
2. Write some gribberish in the rule
3. Try to create the rule

## Your Environment
* Graylog Version: 5.1