Report AssetNotFound errors for the correct asset.

sdk/lib/_internal/pub/lib/src/barback/server.dart

Index: sdk/lib/_internal/pub/lib/src/barback/server.dart
diff --git a/sdk/lib/_internal/pub/lib/src/barback/server.dart b/sdk/lib/_internal/pub/lib/src/barback/server.dart
index f5fe5a10155b93c36129b4fe1eb447915b04241c..a324ecbf96ccf5ba187d4e4d53fcf5d0d8a696de 100644
--- a/sdk/lib/_internal/pub/lib/src/barback/server.dart
+++ b/sdk/lib/_internal/pub/lib/src/barback/server.dart
@@ -131,6 +131,10 @@ class BarbackServer {
         request.response.statusCode = 302;
         request.response.headers.add('location', '${request.uri}/');
         request.response.close();
+      }).catchError((newError, newTrace) {
+        // If we find neither the original file or the index, we should report
+        // the error about the original to the user.
+        throw newError is AssetNotFoundException ? error : newError;
       });
     }).catchError((error, trace) {
       if (error is! AssetNotFoundException) {