on missing tmux, libtmux crashes

[tshirtman]

  File "/home/gabriel/.local/lib/python3.5/site-packages/libtmux/server.py", line 426, in new_session
    if self.has_session(session_name):
  File "/home/gabriel/.local/lib/python3.5/site-packages/libtmux/server.py", line 326, in has_session
    proc = self.cmd('has-session', '-t%s' % target_session)
  File "/home/gabriel/.local/lib/python3.5/site-packages/libtmux/server.py", line 100, in cmd
    return tmux_cmd(*args, **kwargs)
  File "/home/gabriel/.local/lib/python3.5/site-packages/libtmux/common.py", line 153, in __init__
    cmd = [which('tmux')]
  File "/home/gabriel/.local/lib/python3.5/site-packages/libtmux/common.py", line 398, in which
    logger.trace(
AttributeError: 'Logger' object has no attribute 'trace'

It seems libtmux uses a non-existing trace method on the logger to log the error. It's probably easy to forget to test this codepath ;)