Fixed a couple of proxies-related unhandled exceptions.

src/objects.cc

Index: src/objects.cc
diff --git a/src/objects.cc b/src/objects.cc
index 24b54c0d4a3710160d5cde7b57d2bb9a732c2a86..a2220df152731341d093e11f71360f70259b02f6 100644
--- a/src/objects.cc
+++ b/src/objects.cc
@@ -4003,7 +4003,7 @@ Maybe<PropertyAttributes> JSProxy::GetPropertyAttributesWithHandler(
     Handle<Object> error = isolate->factory()->NewTypeError(
         MessageTemplate::kProxyPropNotConfigurable, handler, name, trap);
     isolate->Throw(*error);
-    return Just(NONE);
+    return Nothing<PropertyAttributes>();
   }
 
   int attributes = NONE;