Trigger debug event on not yet caught exception in promises.

src/promise.js

Index: src/promise.js
diff --git a/src/promise.js b/src/promise.js
index 27890a7626b37d12af837e73627617d6a94da4f2..41a5f511921dc64760ab0ce6d5717ba3a7c54cc5 100644
--- a/src/promise.js
+++ b/src/promise.js
@@ -192,8 +192,14 @@ function PromiseHandle(value, handler, deferred) {
     else
       deferred.resolve(result);
   } catch(e) {
-    // TODO(rossberg): perhaps log uncaught exceptions below.
-    try { deferred.reject(e) } catch(e) {}
+    if (GET_PRIVATE(deferred.promise, promiseOnReject).length == 0) {
+      // There is no reject handler defined (yet).  The debugger should know.
+      %DebugPendingExceptionInPromise(e, deferred.promise);
+    }
+    try { deferred.reject(e) } catch(e) {
+      // Reject handler threw (must have been a custom one).
+      %DebugPendingExceptionInPromise(e, deferred.promise);
+    }
   }
 }