Fixes "500pxUser.lua:154: attempt to perform arithmetic on local 'nPhotos' (a nil value)"

The error suggests that the total_items returned from the API is non-numeric in some cases, maybe due to network conditions. This checks that it is numeric and if not shows a more sensible error message.

There are also changes here to switch to listing 50 photos at a time instead of just one. This reduces the http traffic required to sync. It also increases the photo size synced to 5 as requested.