fix: Handle commit errors gracefully

This Merge Request improves the message that we provide to the user when a commit fails by handling several error types individually.

Screenshots

Error type	Message
User session expired
Push rule violation
Branch already exists
File has changed
Unauthorized to push to branch

How to reproduce

There are (of course) different reproduction steps for every error:

Session expired

1. Uncomment the following snippet !151 (diffs)
2. Change a file and commit as usual. That code snippet will delete the authorization token after three seconds which reproduces the session expired error.

Push rule violation

1. Create a push rule in your test project in Project Settings -> Repository -> Push rules. For example: https://gdk.test:3443/gitlab-org/gitlab-shell/-/settings/repository#js-push-rules
2. Create a commit that violates the push rule in the Web IDE.

Branch already exists

1. Create a new branch with a name already taken by another branch.

File has changed

1. Open the Web IDE in two tabs.
2. Open the same branch in both tabs.
3. Change a file in tab no. 1 and commit.
4. Change the same file in tab no. 2 and commit.

Unauthorized to push to that branch

1. Try committing to a branch that you can’t push to like main or master.