"We generally recommend Errors & Omissions Insurance be at the foundation of every company’s insurance portfolio. Usually it is wise to purchase the coverage prior to product launch, or when you have customers. It can be required by investors, particularly VC’s."
Insuring against events that are likely to occur to every insurance holder simultaneously is much more difficult. AIG, for instance, collapsed when lots of people called in their "mortgage value insurances"; a more paranoid underwriter would need to sit on a huge pool of money until something happened.
Other constructions are possible, but a patent troll targeting one small developer is likely to target so many small developers that actually fighting all those lawsuits would bankrupt everyone. You'd need a few big fishes in the pool to absorb those kind of shocks, and I think the Googles of this world prefer fighting their own battles.