Show nicer errors on some server failures.

lib/coreimpl/future_implementation.dart

Index: lib/coreimpl/future_implementation.dart
diff --git a/lib/coreimpl/future_implementation.dart b/lib/coreimpl/future_implementation.dart
index 328c88964c6be89f44914b7bdddb32a12102758e..999013ec5bd198180966c7a283ef84c50fcbfbb1 100644
--- a/lib/coreimpl/future_implementation.dart
+++ b/lib/coreimpl/future_implementation.dart
@@ -169,22 +169,40 @@ class FutureImpl<T> implements Future<T> {
     _complete();
   }
 
-  Future transform(Function transformation) {
+  Future transform(Function transformation, [onException(Object exception)]) {
     final completer = new Completer();
-    handleException((e) {
-      completer.completeException(e, this.stackTrace);
+
+    transformException(ex, stackTrace) {
+      var result = false;
+      if (onException != null) {
+        try {
+          result = onException(ex);
+        } catch (innerException) {
+          ex = innerException;
+        }
+      }
+
+      if (result != true) {
+        completer.completeException(ex, stackTrace);
+      }
+    }
+
+    handleException((ex) {
+      transformException(ex, this.stackTrace);
       return true;
     });
+
     then((v) {
       var transformed = null;
       try {
         transformed = transformation(v);
       } catch (ex, stackTrace) {
-        completer.completeException(ex, stackTrace);
+        transformException(ex, stackTrace);
         return;
       }
       completer.complete(transformed);
     });
+
     return completer.future;
   }