Report syntax errors in natives when building with mksnapshot.

src/mksnapshot.cc

Index: src/mksnapshot.cc
===================================================================
--- src/mksnapshot.cc	(revision 11679)
+++ src/mksnapshot.cc	(working copy)
@@ -302,7 +302,13 @@
   }
 #endif
   i::Serializer::Enable();
+  TryCatch try_catch(NULL);
   Persistent<Context> context = v8::Context::New();
+  if (try_catch.HasCaught()) {
+    fprintf(stderr,
+            "\nException thrown while compiling natives - see above.\n\n");
+    exit(1);
+  }
   ASSERT(!context.IsEmpty());
   // Make sure all builtin scripts are cached.
   { HandleScope scope;