If I understand well what you're proposing, you want to charge a monthly fee to give your costumer the commitment that you will fix some kinds of problems he might have, i.e. "crashes for any non self-inflicted reason". That is basically a specific kind of support contract, widely adopted in the field, and there's nothing wrong with the idea, I just don't see why call it "insurance". As long as what you want to cover in your support contract is clear, all should be fine and you should definitely sell it. If you already have a support contract that covers other problems, you can just offer an extension of the support for extra cases, at an additional cost, that the costumer can choose to hire or not. I would advise you to hire an IT lawyer to help polish the details, if you foresee having many costumers that you could offer this "support extension" to.