Add diagnostic message if external blob files cannot be loaded.

src/snapshot/natives-external.cc

Index: src/snapshot/natives-external.cc
diff --git a/src/snapshot/natives-external.cc b/src/snapshot/natives-external.cc
index a97b0a74423f4001de8a4db79f41cca04adf5016..6505d15571e870ae3edf70859890cbd1e38ea474 100644
--- a/src/snapshot/natives-external.cc
+++ b/src/snapshot/natives-external.cc
@@ -119,11 +119,11 @@ template<NativeType type>
 class NativesHolder {
  public:
   static NativesStore* get() {
-    DCHECK(holder_);
+    CHECK(holder_);
     return holder_;
   }
   static void set(NativesStore* store) {
-    DCHECK(store);
+    CHECK(store);
     holder_ = store;
   }
   static bool empty() { return holder_ == NULL; }