Show nicer errors on some server failures.

lib/core/future.dart

 // Copyright (c) 2011, 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.
 
 /**
  * A [Future] is used to obtain a value sometime in the future.  Receivers of a
  * [Future] can obtain the value by passing a callback to [then]. For example:
  *
  *     Future<int> future = getFutureFromSomewhere();
  *     future.then((value) {
@@ skipping 86 matching lines @@
    * A future representing [transformation] applied to this future's value.
    *
    * When this future gets a value, [transformation] will be called on the
    * value, and the returned future will receive the result.
    *
    * If an exception occurs (received by this future, or thrown by
    * [transformation]) then the returned future will receive the exception.
    *
    * You must not add exception handlers to [this] future prior to calling
    * transform, and any you add afterwards will not be invoked.
+   *
+   * The [onException] parameter can be provided to control how exceptions
+   * received by this future or the transformation are handled. If an exception
+   * occures, [onException] will be called. If that throws an exception, the
+   * returned future will complete with that exception. If it returns `true`,
+   * then the exception is considered handled and is not propogated to the
+   * returned future. If it returns anything else, the original exception will
+   * be propogated.
    */
-  Future transform(transformation(T value));
+  Future transform(transformation(T value), [onException(Object exception)]);
 
    /**
     * A future representing an asynchronous transformation applied to this
     * future's value. [transformation] must return a Future.
     *
     * When this future gets a value, [transformation] will be called on the
     * value. When the resulting future gets a value, the returned future
     * will receive it.
     *
     * If an exception occurs (received by this future, thrown by
@@ skipping 100 matching lines @@
       future.handleException((exception) {
         if (!result.isComplete) {
           completer.completeException(exception, future.stackTrace);
         }
         return true;
       });
     }
     return result;
   }
 }