If you set the default-path option for tmux, new panes won't be in the correct directory. To fix this, you cd to vim's working directory when a new pane is created.
If you run a command outside of tmux (maybe by accident) it explodes with output. To fix, you check the $TMUX environment variable and throw an exception if you aren't in a tmux session.
ensure new panes are in the correct directory
don't explode if you aren't in a tmux session
This looks great, thanks for the pull request! Running cd before any command in the runner pane complements the recent addition of using an existing tmux pane by ensuring that pane has the correct working directory.