Add an inbuilt data litmus test

[emilymclean]

If something fails during data load, it can result in the app becoming functionally unusable until the next day. Adding a "clear cache" button is a good stopgap fix, but it would be nice to perform some basic data integrity checks at launch (and maybe other fixed points) to determine if any data types should be reloaded.

For example, if there are no stops or routes produced by the load call, it can be safe to assume that something went wrong and avoid considering that value as cached.