Fix: sessiond: application channel creation failure stops start cmd

The creation of an application's channel can fail when, for instance,
a context can't be created. This causes applications that would have
been started _after_ it to never be started.

This keeps the iteration going on error and starts all applications
that could be started. This is more in line with the behaviour of
2.10 (and earlier) since those channel creations would occur as
applications registered and not on tracing "start".

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>