`Subprocess.wait_for_exit` never resolves if process terminated before it is called

The wait_for_exit method eventually calls os.waitpid, which throws a ChildProcessError if no process with the specified pid exists. This exception is caught, the function just returns, and the Future never resolves.

if __name__ == '__main__':
    import asyncio
    from tornado.process import Subprocess

    async def f():
        p = Subprocess("ls")
        p.proc.wait()
        return await p.wait_for_exit()

    loop = asyncio.get_event_loop()
    loop.run_until_complete(f())

https://github.com/tornadoweb/tornado/blob/65a9e48f8ce645f104e3e0aa772222e70b0376d9/tornado/process.py#L348

Instead, the process could be retrieved from the _waiting dict and the return code could be accessed from the object directly.

Start in tornado/process.py at Subprocess.wait_for_exit and inspect how os.waitpid errors are handled and how the _waiting dictionary stores processes. Reproduce the example, then make the Future resolve when the process terminated before wait_for_exit was called, using the available return code.