Start better error reporting for sockets

runtime/bin/socket.dart

Index: runtime/bin/socket.dart
diff --git a/runtime/bin/socket.dart b/runtime/bin/socket.dart
index ad48172bf596e8a03ec49c1e9a1a77480b198405..219a7aa1c55b0363d0e1a977f8582db121012b5f 100644
--- a/runtime/bin/socket.dart
+++ b/runtime/bin/socket.dart
@@ -18,7 +18,7 @@ interface ServerSocket default _ServerSocket {
   /**
    * The error handler gets called when a socket error occurs.
    */
-  void set onError(void callback());
+  void set onError(void callback(Exception e));
 
   /**
    * Returns the port used by this socket.
@@ -88,7 +88,7 @@ interface Socket extends Hashable default _Socket {
   /**
    * The error handler gets called when a socket error occurs.
    */
-  void set onError(void callback());
+  void set onError(void callback(Exception e));
 
   /**
    * Returns input stream to the socket.
@@ -122,7 +122,21 @@ interface Socket extends Hashable default _Socket {
 
 
 class SocketIOException implements Exception {
-  const SocketIOException([String this.message = ""]);
-  String toString() => "SocketIOException: $message";
+  const SocketIOException([String this.message = "",
+                           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");
+    }
+    return sb.toString();
+  }
   final String message;
+  final OSError osError;
 }