tmux config errors are hidden

Distribution: Ubuntu 21.10, x64

byobu package version: 5.133-0ubuntu1
tmux package version: 3.1c1build1

Steps to reproduce:

Configure byobu to use tmux.
Ensure there is no tmux server running with socket "/tmp/tmux-$UID/default".
Add a syntax error to "$HOME/.byobu/windows.tmux".
In a terminal window, run "byobu".

Observed results:
tmux exits with an error. The tmux error message is suppressed, and the only visible output on the terminal is "no server running on /tmp/tmux-$UID/default".

Expected results:
The error messages from both tmux and byobu are visible in the terminal.

Workaround: Run "tmux new" in a different terminal before running "byobu". If the tmux server is already running, then byobu will correctly display the tmux errror message, allowing the user to diagnose and correct the error.