Improve error message when branch creation fails due to naming conflicts

Proposal

When attempting to create a branch with a hierarchical name (e.g., release/123) while a parent branch (e.g., release) already exists, GitLab displays a misleading error message: Failed to create branch 'release/123': invalid reference name 'main'.

This error incorrectly suggests the problem is with the 'main' branch, when the actual issue is a Git limitation regarding branch naming conflicts.

A clear, informative error message should be raised instead like:

Cannot create branch 'release/123' because branch 'release' already exists.
Git does not allow a branch to be both a file and a directory.

Recreation

1. Create a new project.
2. Create a new branch from main named release.
3. Create a new branch from main named release/123.
4. Observed this error message.

Failed to create branch 'release/123': invalid reference name 'main'