parasite-syscall: describe task's state on trap failure

I think we can improve error messages to distinguish task's kills
from segfaults from ptrace failures & etc.

Signed-off-by: Dmitry Safonov <dsafonov@virtuozzo.com>
Acked-by: Andrew Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>