Display error message when the pipeline rule title already exists

[gally47]

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Description

As described in this issue https://github.com/Graylog2/graylog2-server/issues/11812 the description was not being saved when creating a pipeline rule but the real reason was testing with the same existing title. So in this PR the error is displayed in a readable way that the user will know why he couldn't create a new pipeline rule.

Motivation and Context

https://github.com/Graylog2/graylog2-server/issues/11812

How Has This Been Tested?

Screenshots (if appropriate):

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [x] Refactoring (non-breaking change)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [X] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [ ] I have read the CONTRIBUTING document.
• [ ] I have added tests to cover my changes.