Change a return value of reactor::poll to io::Result. (#40)

* Change a return value of reactor::poll to io::Result.

* Revert "Change a return value of reactor::poll to io::Result."

This reverts commit 281d8c32d4.

* Return a result from reactor::poll.

* Drop a reactor if any error occurs. Fix warnings in tests.

* Update a documentation for reactor::turn.

* Unwrap the last turn() call in tests.

src/reactor/global.rs

@@ -49,6 +49,6 @@ impl Drop for HelperThread {
 
 fn run(mut reactor: Reactor, done: Arc<AtomicBool>) {
     while !done.load(Ordering::SeqCst) {
-        reactor.turn(None);
+        reactor.turn(None).unwrap();
     }
 }