Fail gracefully when trying to trace if LTTng is not installed

This adds a check to make sure that the current platform can support LTTng and that it is installed before trying to trace with ros2trace or tracetools_trace. If LTTng is not found, it exits (error code 2) and prints an error message which includes a link to this repo.

It also adds a call to ros2 trace (expecting it to fail) as part of the no_lttng job.

Closes #93 (closed)