[kernel][spawn service] Avoid logging of unecessary exception.

When killing an agent, if the agent does not exist (never launched or
already killed), the killAgent function does not log an error any more.
It is assumed that when the killAgent function is called, if the agent
is already killed, then the function should not fail.

see #84

Signed-off-by: Stéphane Galland <galland@arakhne.org>