Add a note explaining why the onerror doesn't generate a more detailed exception (dart-lang/web_socket_channel#273)

pkgs/web_socket_channel/lib/html.dart

diff --git a/pkgs/web_socket_channel/lib/html.dart b/pkgs/web_socket_channel/lib/html.dart
index a375480..25b4175 100644
--- a/pkgs/web_socket_channel/lib/html.dart
+++ b/pkgs/web_socket_channel/lib/html.dart
@@ -99,6 +99,8 @@
     // The socket API guarantees that only a single error event will be emitted,
     // and that once it is no open or message events will be emitted.
     innerWebSocket.onError.first.then((_) {
+      // Unfortunately, the underlying WebSocket API doesn't expose any
+      // specific information about the error itself.
       final error = WebSocketChannelException('WebSocket connection failed.');
       _readyCompleter.completeError(error);
       _controller.local.sink.addError(error);