Fix: Improve the error reporting of the track/untrack command

This patch adds feedback when a track/untrack command is successful and
specific messages on errors. It also changes the behavior when tracking
an already tracked PID or untracking a PID that is not tracked from an
error to a warning with a successful exit code.

Fixes: #898
Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>