Handle webhook notifications for unknown users

[caarmen]

Instead of returning a 500 and logging a traceback to the console: return a 404.

[github-actions]

File	Coverage
All files	98%	✅
logger.py	90%	✅
main.py	97%	✅
domain/localrepository/localwithingsrepository.py	94%	✅
domain/usecases/slack/usecase_post_sleep.py	97%	✅
remoteservices/api/withings/subscribeapi.py	83%	✅
remoteservices/api/withings/weightapi.py	93%	✅
remoteservices/repositories/webapifitbitrepository.py	97%	✅
routers/dependencies.py	85%	✅
tasks/fitbitpoll.py	96%	✅

Minimum allowed coverage is 80%

Generated by 🐒 cobertura-action against 6659fa4