Creating Pipeline Rule with non-unique name results in cryptic error

## Expected Behavior

When trying to create a Pipeline Rule with a name that already exists, a hint should be shown and the "Create" button should be disabled.

## Current Behavior

When entering a title for a Pipeline Rule that already exists, I can still click the "Create" button, but the API request fails. An error notification with a cryptic error is shown, not helping the user to know that a unique title needs to be used.

## Possible Solution

The title should be validated in the regular form validation as well, showing an error next to it when it has already been taken. This should be repeated when clicking the "Create" button to avoid race conditions (e.g. a rule with the same name had been created since the last validation run).

## Steps to Reproduce (for bugs)

1.
2.
3.
4.

## Context

## Your Environment

* Graylog Version:
* Java Version:
* OpenSearch Version:
* MongoDB Version:
* Operating System:
* Browser version: