You can get a general liability insurance policy, but you will probably need to have your Terms of Service and Accepted Use Policy set beforehand. And those documents are probably the most important things, even more important than general liability insurance.
No one guarantees against data loss (at least in the hosting world). Read the Terms of Service of some providers to get an idea.
You also will want to limit any monetary guarantees to actual money paid, pro-rata for the time down, as verified by independent monitoring agency -- otherwise, when a cable cut happens between you and your PITA client you will end up paying for the site being "down".
People will pay you $4 a month for hosting, then insist they lost $25,000 when you were down for ten minutes. You have to plan for the jerks, so have a good Terms of Service.
Terms of service is not bullet proof in court. Get insurance, if anything it will cover legal defense costs when you get sued (it will happen eventually).
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