Fix an edge case.
Fix an edge case.

When sponges is use in foreground, and supervisor receive a kill signal,
it properly exits. But, the runner responsible of keeping sponges in
foreground will end up trying kill the supervisor on a "Ctrl-C". But,
the supervisor is already gone, and so ruby raise an "Errno::ESRCH"
exception.

Signed-off-by: chatgris <f9469d12bf3d131e7aae80be27ccfe58aa9db1f1@af83.com>

AF83/sponges,sr-education/sponges,AF83/sponges