148f46620f
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>