The router prints error no matter what error type it is.
It's useful for troubleshooting, however, it also prints
the context canceled error, which means that users abort
request before reply and its really normal nowadays. Also,
the router returns 502 if error is not nil and may confused
client if it's a timeout error.
To solve these problems, this PR adds an error handler to
reverse proxy to examine the return error and change the
status code when needed.
