TNL-782 each request should have valid ID Token

Each incoming request is expected to have signed non-expired JSON Web Token, containing information about user (sub), matching edx-notes client id (to be set in provider's database) and signed with edx-notes client secret (the same). 

https://openedx.atlassian.net/browse/TNL-782
if these cases need to be logged at all, i'm not sure `exception` is the best level.   any reason not to do `logger.debug` as you're doing in other places?