According to EU rules, a flight which is diverted to another airport, which does not serve the same town, city or region as the airport for which the booking was made, has to be treated as a cancelled flight. By landing in Bologna instead of Venice the airline cancelled your flight to Venice, since the Bologna airport does not serve the same town, city or region as the Venice airport. You have the same rights as in case of a flight cancellation; in addition, you can claim the costs you incurred for getting to Venice.
Had the airline provided for your onward transport to Venice, e.g. by bus or taxi, the flight would have been considered delayed
This.
The destinations given are obviously different, but this is taken from the EU website. According to this, Ryanair effectively cancelled your flight, so you can make a claim for this, plus expenses. I suppose the only woolly area is whether they will say the 'cancellation' was due to extraordinary circumstances (weather), but I find it hard to believe they don't still have a responsibility to get you to your final destination, whatever the circumstances of the diversion.
This is exactly what travel insurance is for
Um. no it's not. Ryanair have a duty of care, it's not for travel insurance to make up for their shortcomings. That said, if you get no joy from Ryanair, you could possibly try your travel insurance.