I have suffered through this a number of times in Indonesia and South America. There's often not much you can do if the only way to get where you're going is via an airport that's had to stop flights. That being said, my experience is that my travel insurance does cover the cost of the aborted trip due to volcanic ash. However you had to have bought the travel insurance prior to the volcano erupting - which one of my travel companions learned the hard way.
I am in the same situation as Mikey Canuk with my trip to Raja Ampat in just over a month. However if my flights to Sorong get delayed and I miss the boat, I already have an established contingency plan in place. I already have existing plans to go to Ambon after Raja Ampat, so all I will do is rearrange my flight from Sorong to Ambon earlier than anticipated, call the resort, and spend that extra time diving in Ambon instead. If your flights do get delayed, it might be a good idea for you to also have a backup plan in place. Sometimes you simply just have to make the best of a bad situation.