Fix: lttng view, error message and exit code

lttng view is a frontend command, when the viewer is not found in the
path, the error message should be human readable (no need for the
developper-oriented debug message).

Also the return code must indicate that something went wrong if the
viewer is not on the system.

(fix #144)

Signed-off-by: Julien Desfossez <julien.desfossez@efficios.com>
Signed-off-by: David Goulet <dgoulet@ev0ke.net>