TUI should gracefully require tmux

[wbxyz]

Some of the commands in the TUI require that it be nested in a tmux session. Currently only the log file viewing requires this, but other features are likely to need this feature.

I think there are three options:

1. If the user is not in a tmux session when TUI is started, launch a tmux session on their behalf.
2. Refuse to start TUI if it's not in a tmux session.
3. Allow TUI to function at best effort and log an error message when someone invokes one of these commands.