The E&O insurance I looked into getting when I moved into consulting would have covered it -- "lost sales" resulting from a "hardware or software malfunction." I assume if you start making recurring claims on that the insurance company will reevaluate whether they want to continue doing business with you, though.
Don't know specifically about "server failure" insurance, but I assume it exists. There is insurance for practically everything. For example, an Uncle of my friend builds home security systems, and he is insured in case a home he has secured is ever broken into anyway.