Add debug logs to encryption prep
This extra debug logs may help isolate the cause of
https://github.com/vector-im/element-web/issues/16194.
These changes also fix a related (but most likely different) failure mode: if a
failure occurred in the `encryptionPreparation` async task, we would skip trying
to prepare in all future attempts for that room. This change ensures prep
failures are logged and we resume prep attempts on the next call from the
application.
