Display a msg when contexts are killed by uncatched exceptions

Display a msg when contexts are killed by uncatched exceptions

and when I want to really kill an actor (eg when its host is turned
off), I launch an uncatchable kernel::Context::StopRequest instead of
a catchable simgrid::HostFailureException (which will be used in case
of remote exec and similar)

Maybe there should be a config flag to decide if we want to kill the
simulation when an actor fails. The current setting forces the user to
add try/catch (simgrid::Exception) around their main functions. That's
not a bad thing either, not sure.