`shallow-backup` crashes if repo can't be created

I attempted to create a brand new backup here. The process here is:

Read the "backup directory" path from the config file.
Check to see if it contains a git repo -- if not, create it.
Do cool backup things
Profit

My config was set to back up to /home/pwn/shallow-backup.

On this machine, that directory was owned by root, and pwn had read access to /home/pwn/shallow-backup but didn't have write access.

And so we see the following exception:

pwn@strawman:~$ shallow-backup
        [graphic removed for readability]
    v5.1 by Aaron Lichtman (@alichtman)

Current shallow-backup path: ~/shallow-backup
[?] Would you like to move this somewhere else?:  No
 >  No
    Yes

Initializing new git repo...
Traceback (most recent call last):
  File "/usr/local/bin/shallow-backup", line 8, in <module>
    sys.exit(cli())
  File "/usr/local/lib/python3.9/dist-packages/click/core.py", line 1130, in __call__
    return self.main(*args, **kwargs)
  File "/usr/local/lib/python3.9/dist-packages/click/core.py", line 1055, in main
    rv = self.invoke(ctx)
  File "/usr/local/lib/python3.9/dist-packages/click/core.py", line 1404, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/usr/local/lib/python3.9/dist-packages/click/core.py", line 760, in invoke
    return __callback(*args, **kwargs)
  File "/usr/local/lib/python3.9/dist-packages/shallow_backup/__main__.py", line 99, in cli
    repo, new_git_repo_created = safe_git_init(backup_home_path)
  File "/usr/local/lib/python3.9/dist-packages/shallow_backup/git_wrapper.py", line 70, in safe_git_init
    repo = git.Repo.init(dir_path)
  File "/usr/local/lib/python3.9/dist-packages/git/repo/base.py", line 1046, in init
    git.init(**kwargs)
  File "/usr/local/lib/python3.9/dist-packages/git/cmd.py", line 639, in <lambda>
    return lambda *args, **kwargs: self._call_process(name, *args, **kwargs)
  File "/usr/local/lib/python3.9/dist-packages/git/cmd.py", line 1184, in _call_process
    return self.execute(call, **exec_kwargs)
  File "/usr/local/lib/python3.9/dist-packages/git/cmd.py", line 984, in execute
    raise GitCommandError(redacted_command, status, stderr_value, stdout_value)
git.exc.GitCommandError: Cmd('git') failed due to: exit code(1)
  cmdline: git init
  stderr: '/home/pwn/shallow-backup/.git: Permission denied'

This case needs clean exception handling.

alichtman / shallow-backup

`shallow-backup` crashes if repo can't be created #308