Print message on a successful load command

Fixes #754

Signed-off-by: David Goulet <dgoulet@efficios.com>
Update load help message

Signed-off-by: David Goulet <dgoulet@efficios.com>
Fix: bad errno handling in load session from path

Signed-off-by: David Goulet <dgoulet@efficios.com>
Fix: allow loading session file with -i option

Signed-off-by: David Goulet <dgoulet@efficios.com>
Fix: update test to use correct lttng load option

Signed-off-by: David Goulet <dgoulet@efficios.com>