Fix error when using invalid branch name when creating a new pipeline

## What does this MR do?

This MR fixes `500` error when creating a new pipeline though user interface ("Run pipeline")

## Are there points in the code the reviewer needs to double check?

Is this a good approach to catch those exceptions on `Repository` level?

## Does this MR meet the acceptance criteria?

- [x] [Changelog entry](https://docs.gitlab.com/ce/development/changelog.html) added
- Tests
  - [x] Added for this feature/bug

## What are the relevant issue numbers?

Closes #23982

See merge request !7324