Improve logging for workspacebuilder

[tony]

in re: debugging cases like #364

Requirements / blockers

Requires improvements for libtmux and its subprocess calls

Details

1. There's no way to assert the message is raised - except maybe checking pytest's caplog with logging turned up to the max.
2. We need better introspection of events at the libtmux level:
   • Being able to trace where in the config / workspacebuilder libtmux methods are being run
     • and the subprocess calls to tmux(1) within those