Liability? Probably none. See section "11. Limitations of Liability." here: https://aws.amazon.com/agreement/

RDS SLA's are here (doesn't mention backups though, so not sure how that's handled): https://aws.amazon.com/rds/sla/

Not a lawyer or anything, but my layman's understanding is that, you essentially are voluntarily opting-in to waiving liability when you sign up for AWS and accept the terms and conditions, and instead of liability, you agree to accept service credits if SLAs are not met.