Improve the error message when an import fails.

runtime/bin/vmservice/loader.dart

Index: runtime/bin/vmservice/loader.dart
diff --git a/runtime/bin/vmservice/loader.dart b/runtime/bin/vmservice/loader.dart
index 9b7355e0b7ccad18fadf2d3fe8961877bdf15d54..7bec4a1cda1c23147d5e19bd897ff7deeea1a677 100644
--- a/runtime/bin/vmservice/loader.dart
+++ b/runtime/bin/vmservice/loader.dart
@@ -57,8 +57,7 @@ void _loadFile(SendPort sp, int id, Uri uri) {
     _sendResourceResponse(sp, id, data);
   },
   onError: (e) {
-    var err = "Error loading $uri:\n  $e";
-    _sendResourceResponse(sp, id, err);
+    _sendResourceResponse(sp, id, e.toString());
   });
 }