Change need_read and need_write to return an error

This commit is targeted at solving tokio-rs/tokio-core#12 and incorporates the
solution from tokio-rs/tokio-core#17. Namely the `need_read` and `need_write`
functions on `PollEvented` now return an error when the connected reactor has
gone away and the task cannot be blocked. This will typically naturally
translate to errors being returned by various connected I/O objects and should
help tear down the world in a clean-ish fashion.

src/reactor/mod.rs

@@ -231,6 +231,19 @@ impl fmt::Debug for Core {
     }
 }
 
+impl Drop for Inner {
+    fn drop(&mut self) {
+        // When a reactor is dropped it needs to wake up all blocked tasks as
+        // they'll never receive a notification, and all connected I/O objects
+        // will start returning errors pretty quickly.
+        let io = self.io_dispatch.read().unwrap();
+        for (_, io) in io.iter() {
+            io.writer.notify();
+            io.reader.notify();
+        }
+    }
+}
+
 impl Inner {
     /// Register an I/O resource with the reactor.
     ///