Extend dart:io error handling to all socket functions

runtime/bin/socket.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 interface ServerSocket default _ServerSocket {
   /**
    * Constructs a new server socket, binds it to a given address and port,
    * and listens on it.
    */
   ServerSocket(String bindAddress, int port, int backlog);
@@ skipping 116 matching lines @@
                            OSError this.osError = null]);
   String toString() {
     StringBuffer sb = new StringBuffer();
     sb.add("SocketIOException");
     if (!message.isEmpty()) {
       sb.add(": $message");
       if (osError != null) {
         sb.add(" ($osError)");
       }
     } else if (osError != null) {
-      sb.add(": osError");
+      sb.add(": $osError");
     }
     return sb.toString();
   }
   final String message;
   final OSError osError;
 }