Don't accept poor-formatted POSTs

[LookItsJosh]

If the app sends a corrupt or manually edited POST with a non-base64 encoded string, it will crash after the backend returns those values. Rejecting non encoded strings ought to fix this issue and prevent additional potential API problems.

[ramnanib2]

can you post an example ? The back-end validates the report ensuring TCN report format is honored.

[LookItsJosh]

A colleague brought it up to me, the client side issue is Co-Epi/app-android#204. It has all the details I have, but I can request more if needed.

Ideally, the frontend can handle an improper response without crashing, but the backend should try to catch improper requests before they are sent back.