RunError and few more error types implements Error (#501)

This allows them to be used with things like `failure`.

src/runtime/current_thread/runtime.rs

@@ -9,6 +9,8 @@ use tokio_executor;
 
 use futures::Future;
 
+use std::fmt;
+use std::error::Error;
 use std::io;
 
 /// Single-threaded runtime provides a way to start reactor
@@ -48,6 +50,21 @@ pub struct RunError {
     inner: current_thread::RunError,
 }
 
+impl fmt::Display for RunError {
+    fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
+        write!(fmt, "{}", self.inner)
+    }
+}
+
+impl Error for RunError {
+    fn description(&self) -> &str {
+        self.inner.description()
+    }
+    fn cause(&self) -> Option<&Error> {
+        self.inner.cause()
+    }
+}
+
 impl Runtime {
     /// Returns a new runtime initialized with default configuration values.
     pub fn new() -> io::Result<Runtime> {