Merge pull request #821 from ethereum/fixImport3

cb(error) if any

src/app.js

@@ -378,6 +378,8 @@ function run () {
         // FIXME: at some point we should invalidate the browser cache
         filesProviders['browser'].addReadOnly(url, content)
         cb(null, content)
+      } else {
+        cb(error)
       }
     })
   })