Tmux session is killed when `before_script` fails while using `--append`

Let me first thank you for this amazing tool! Running tmuxp load has quickly become the very first thing I do when I return to a project.

However, I've noticed the following rather annoying bug.

Let's say we have the following workspace file .tmuxp.yaml.:

session_name: foo
before_script: /bin/false
windows:
  - window_name: bar
    panes:
      - date

When we create a tmuxp session with tmuxp load .tmuxp.yaml it errors out as expected:

tmuxp.exc.BeforeLoadScriptError: before_script failed with returncode 1.
command: /bin/false
Error output:

So far, so good. However, if we first create a Tmux session with a couple of windows:

tmux new-session -d -s dev -n first_window
tmux new-window -t dev -n second_window
tmux new-window -t dev -n third_window
tmux attach -t dev

and then create the tmuxp session with the --append flag: tmuxp load --append .tmuxp.yaml, I get only [exited] as output, and my entire Tmux session (including existing windows) is gone.

Hopefully you can reproduce this. Thanks again for tmuxp!

Start with the tmuxp load --append entry point and the handling of before_script failures. Reproduce the issue using the .tmuxp.yaml example and the listed tmux commands; done means the failure still reports the error while the existing session and windows remain intact.