api_server: improve termination message
api_server: improve termination message

Turn `unwrap` into `expect` for a more friendly
and frequently encountered exit message.

Signed-off-by: Diana Popa <4240422768c1b315e51818a074064501037757cd@amazon.com>

firecracker-microvm/firecracker,firecracker-microvm/firecracker,firecracker-microvm/firecracker,firecracker-microvm/firecracker,firecracker-microvm/firecracker