gen_snapshot: When copying an error across isolates, remember to turn it back into an error.

runtime/bin/gen_snapshot.cc

Index: runtime/bin/gen_snapshot.cc
diff --git a/runtime/bin/gen_snapshot.cc b/runtime/bin/gen_snapshot.cc
index beb89f9e7c2e462a3293e7c2a3f2988b09c9ad6c..732493d5fe11dd42cf3c9cdf7dad25252f531a6f 100644
--- a/runtime/bin/gen_snapshot.cc
+++ b/runtime/bin/gen_snapshot.cc
@@ -448,7 +448,7 @@ static Dart_Handle LoadUrlContents(const char* uri_string) {
     result = Dart_NewStringFromUTF8(payload, payload_length);
     free(payload);
   } else {
-    result = DartUtils::NewString(result_string);
+    result = Dart_NewApiError(result_string);
     free(result_string);
   }
   return result;