init.sh: Abort on errors and show more debug infos

When viewing the container start logs, we want to see which step failed exactly.
And if one fails, the script should not continue to run, as this leads to weird
follow-up failures. Motivation for that change:

- Cloning the repos failed due to a bad token
- The `cd submission_repo` failed (as it doesn't exist)
- The `cd ..` thus moved out of the project directory
- And thus, starting gunicorn failed with `ModuleNotFoundError: No module named
  'flask_app'` after a long traceback, easily hiding the actual error.

Setting `-e` required making the rest of the script run fine no matter what
state things are in (local student volume and branch on GitLab).

Hopefully, it will be enough to:

- Not clone the repo if it already exists
- Deal with the upstream branch not existing yet