Review application behavior for null file, or unparseable file

[bradyt]

The last time I was running cone in an iPhone Simulator, I think I saw CPU usage when the app showed loading spinner with file as null. Maybe the fix could become obvious, but I should try to be a little bit systematic in how I discover this. I'm not sure if there's a way to automate a test for this.

There may be similar behavior for a file that cone can not parse, but I haven't researched yet. I notice that the loading spinner will appear as it apparently attempts to parse.