On Unix, tokio::process::ExitStatus::code() will return None if the
process was terminated by a signal. So handle to avoid panicking
caused by unwrap().
Signed-off-by: Liu Jiang <gerry@linux.alibaba.com>
Enhance documentation for ProcessMonitor, to explicitly state the
requirement of ProcessMonitor::start().
Signed-off-by: Liu Jiang <gerry@linux.alibaba.com>