## Problem
When a user creates a branch but their GitHub account does not have
access to the repo, the UI shows a generic branch not found message.
## Solution
The API actually returns a more detailed message. Show it instead of the
generic one.
<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit
* **Bug Fixes**
* Fixed error message handling when creating branches from invalid Git
references. Users will now receive appropriate error messages when
attempting to use a non-existent branch.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->
Gildas Garcia
60992fd8d8