make App.open_for_* look more like io.open

Now missing files will result in similar behavior: nil file handles.

app.lua

@@ -302,6 +302,7 @@ function App.open_for_writing(filename)
 end
 
 function App.open_for_reading(filename)
+  if App.filesystem[filename] then
     return {
       lines = function(self)
                 return App.filesystem[filename]:gmatch('[^\n]+')
@@ -309,6 +310,7 @@ function App.open_for_reading(filename)
       close = function(self)
               end,
     }
+  end
 end
 
 function App.run_tests()