Catch when CryptoClient can't find room members

[AndrewFerr]

CryptoClient#onRoomEvent may throw when looking up the members of a room with a client that isn't in the room, which can happen if appservice namespaces are broad & the appservice receives events for rooms that some of its managed clients are not members of.

Checklist

• [ ] Tests written for all new code
  • Not adding a test for this until there are tests for appservice crypto.
• [x] Linter has been satisfied