Asset load failures in Barback should produce AssetNotFoundExceptions.

pkg/barback/test/package_graph/errors_test.dart

Index: pkg/barback/test/package_graph/errors_test.dart
diff --git a/pkg/barback/test/package_graph/errors_test.dart b/pkg/barback/test/package_graph/errors_test.dart
index d1e8ea3726f579dfff1e24bfce519b9650eed7e7..fed8ea54514c8d8027403897a74a247c485ca37f 100644
--- a/pkg/barback/test/package_graph/errors_test.dart
+++ b/pkg/barback/test/package_graph/errors_test.dart
@@ -146,10 +146,20 @@ main() {
   });
 
   test("an asset isn't passed through a transformer with an error", () {
-    initGraph(["app|foo.txt",], {"app": [[new BadTransformer([])]]});
+    initGraph(["app|foo.txt"], {"app": [[new BadTransformer([])]]});
 
     updateSources(["app|foo.txt"]);
     expectNoAsset("app|foo.txt");
     buildShouldFail([isTransformerException(equals(BadTransformer.ERROR))]);
   });
+
+  test("a transformer can catch an error loading a secondary input", () {
+    initGraph(["app|foo.txt"], {"app": [
+      [new CatchAssetNotFoundTransformer(".txt", "app|nothing")]
+    ]});
+
+    updateSources(["app|foo.txt"]);
+    expectAsset("app|foo.txt", "failed to load app|nothing");
+    buildShouldSucceed();
+  });
 }