[web] improve handling of non-existing olm sessions
Summary: Previous code versions instead of No session for device... was causing runtime error: Cannot destructure property 'session' of cryptoStore.contentSessions [deviceID]..
Test Plan:
- Check encrypt/decrypt when session exists
- Check encrypt/decrypt when session not exists
Reviewers: tomek, marcin
Reviewed By: marcin
Subscribers: ashoat
Differential Revision: https://phab.comm.dev/D12044