archiver-bot: fix MegolmEvent attr crash, propagate sync errors
MegolmEvent has no `.message` attribute (the real field is `.ciphertext`),
so the on_megolm callback added in 54d7388 raised AttributeError on the
first undecryptable event and took the bot down. Drop the field from the
log line — sender + session_id are enough.
Also re-raise after logging in the sync_forever wrapper so the process
exits non-zero and systemd's Restart=on-failure actually triggers; the
swallowed exception was why the bot stayed dead for 5 days instead of
restarting.