When the homeserver returns an error response, matrix-nio parses it
as a SyncError return value rather than raising an exception. The sync
loop only had backoff in the except handler, so SyncError caused a
tight retry loop (~489 req/s) flooding logs and hammering the
homeserver. Check the return value and sleep 5s before retry.
Cherry-picked from PR #2937 by ticketclosed-wontfix.
Co-authored-by: ticketclosed-wontfix <ticketclosed-wontfix@users.noreply.github.com>
148f46620f