rust-chg: suppress panic while writing chg error to stderr

Otherwise "chg >/dev/full 2>&1" would exit with 101. Spotted by test-basic.t.

rust/chg/src/main.rs

@@ -1,59 +1,59 b''
 // Copyright 2018 Yuya Nishihara <[e-mail]>
 //
 // This software may be used and distributed according to the terms of the
 // GNU General Public License version 2 or any later version.
 
 extern crate chg;
 extern crate futures;
 extern crate tokio;
 extern crate tokio_hglib;
 
 use chg::{ChgClientExt, ChgUiHandler};
 use chg::locator;
 use chg::procutil;
 use futures::sync::oneshot;
 use std::env;
 use std::io;
 use std::process;
 use tokio::prelude::*;
 use tokio_hglib::UnixClient;
 
 fn main() {
     let code = run().unwrap_or_else(|err| {
-        eprintln!("chg: abort: {}", err);
+        writeln!(io::stderr(), "chg: abort: {}", err).unwrap_or(());
         255
     });
     process::exit(code);
 }
 
 fn run() -> io::Result<i32> {
     let current_dir = env::current_dir()?;
     let sock_path = locator::prepare_server_socket_path()?;
     let handler = ChgUiHandler::new();
     let (result_tx, result_rx) = oneshot::channel();
     let fut = UnixClient::connect(sock_path)
         .and_then(|client| {
             client.set_current_dir(current_dir)
         })
         .and_then(|client| {
             client.attach_io(io::stdin(), io::stdout(), io::stderr())
         })
         .and_then(|client| {
             let pid = client.server_spec().process_id.unwrap();
             let pgid = client.server_spec().process_group_id;
             procutil::setup_signal_handler_once(pid, pgid)?;
             Ok(client)
         })
         .and_then(|client| {
             client.run_command_chg(handler, env::args_os().skip(1))
         })
         .map(|(_client, _handler, code)| {
             procutil::restore_signal_handler_once()?;
             Ok(code)
         })
         .or_else(|err| Ok(Err(err)))  // pass back error to caller
         .map(|res| result_tx.send(res).unwrap());
     tokio::run(fut);
     result_rx.wait().unwrap_or(Err(io::Error::new(io::ErrorKind::Other,
                                                   "no exit code set")))
 }