First, a correction. Israel is thought to have around 150 nuclear warheads. None, so far as I'm aware, are mounted on missiles. All, that I know of, are supposed to be gravity bombs.
Second, Israel's weapons are almost certainly disbursed at a variety of sites (airfields) around the country. That way, any strike will not eliminate their ability to launch a counter strike.
Third, nuclear weapons are very complex, and they are very unlikely to detonate unless meant to do so. Unless armed in precisely the correct sequence, nuclear bombs can (and have) fallen from aircraft, or crashed inside of aircraft carrying them, and not detonated.
Finally, if Iran were, hypothetically, to launch a nuclear missile attack on Israel, the Israeli defense forces would have at least some home of knocking the missile down. Moreover, due to both US and Israeli early warning systems, it's fairly likely that they would have nuclear armed aircraft up and on their way to Iran long before the missile struck home. Lastly, is such an event, I consider it at least somewhat likely that Israel would strike its most threatening Arab neighbors at the same time as launching the counter strike. As such, one could say that there would be serious fallout for the Syrians and other Arab neighbors of Israel and allies of Iran in the event of an Iranian first strike.