Display a warning message in the terminal in red if starting the session fails because the binary does not exist or if the shell crashes or exits with a non-zero status code.

In these cases, also prevent the tab from being closed automatically.

If the binary for the session cannot be found, display a warning and try to fall back to $SHELL or as a last attempt, /bin/sh.

This should mean that users should always be given a semi-functional basic environment if their profiles get messed up, the shell program gets uninstalled etc.