Report promise reject with no handler (behind a flag).

src/objects-printer.cc

Index: src/objects-printer.cc
diff --git a/src/objects-printer.cc b/src/objects-printer.cc
index d709a207c1d3be7802d9610a837ee831243bc958..2ab7c45f4a4c1f218e3cffadd01c583cec3aeb3a 100644
--- a/src/objects-printer.cc
+++ b/src/objects-printer.cc
@@ -563,6 +563,7 @@ void JSMessageObject::JSMessageObjectPrint(OStream& os) {  // NOLINT
   HeapObject::PrintHeader(os, "JSMessageObject");
   os << " - type: " << Brief(type());
   os << "\n - arguments: " << Brief(arguments());
+  os << "\n - promise: " << Brief(promise());
   os << "\n - start_position: " << start_position();
   os << "\n - end_position: " << end_position();
   os << "\n - script: " << Brief(script());