Fail gracefully when LTTng is not installed

Currently, a user is able to install/build ros2trace, but if they do not have LTTng installed then they get this ugly traceback at runtime:

Traceback (most recent call last):
  File "/opt/ros/foxy/bin/ros2", line 11, in <module>
    load_entry_point('ros2cli==0.9.2', 'console_scripts', 'ros2')()
  File "/opt/ros/foxy/lib/python3.8/site-packages/ros2cli/cli.py", line 67, in main
    rc = extension.main(parser=parser, args=args)
  File "/opt/ros/foxy/lib/python3.8/site-packages/ros2trace/command/trace.py", line 30, in main
    init(
  File "/opt/ros/foxy/lib/python3.8/site-packages/tracetools_trace/trace.py", line 66, in init
    lttng.lttng_init(
  File "/opt/ros/foxy/lib/python3.8/site-packages/tracetools_trace/tools/lttng.py", line 65, in lttng_init
    trace_directory = _lttng.setup(
  File "/opt/ros/foxy/lib/python3.8/site-packages/tracetools_trace/tools/lttng_impl.py", line 81, in setup
    subprocess.run(
  File "/usr/lib/python3.8/subprocess.py", line 489, in run
    with Popen(*popenargs, **kwargs) as process:
  File "/usr/lib/python3.8/subprocess.py", line 854, in __init__
    self._execute_child(args, executable, preexec_fn, close_fds,
  File "/usr/lib/python3.8/subprocess.py", line 1702, in _execute_child
    raise child_exception_type(errno_num, err_msg, err_filename)
FileNotFoundError: [Errno 2] No such file or directory: 'lttng-sessiond'

I think it would be much better if the CLI tool would detect the missing dependency and inform the user what they should install (e.g. an apt install suggestion or a link to some documentation).

I haven't tried on macOS or Windows, but I think we should make sure that the tool also fails gracefully on these platforms.