From the time the volcano first erupted, there was concern about air traffic safety, and yet no effort to avert the serious consequences of continued eruption and spread of ash into the jet stream on international travel seems to have been made. Airlines went right on scheduling flights, and moving people around, without warning and without regard for what was (very likely) coming. Governments made no attempt to plan for alternative transport, for housing and feeding large numbers of tourists, etc.
Could planning have helped? Of course. The effect of volcanic ash on aircraft engines was already known (hence the early concern.) Meteorologists were well aware of the path the ash cloud would take at different levels of the atmosphere. Airlines could have foreseen (and governments should have told them, if they did not) that flights would be restricted. Flight changes and cancellations should have begun well before need, with refunds to all affected passengers, so that more tourists didn't end up in northern European airports during the eruption.
Meteorologists could have suggested the nearest (to Europe) airfields that would not be affected (such as NW Africa, for a start) and fuel could have been moved there in advance of need, with flights re=routed from northern Europe to southern Europe (first) and then Africa (second) for trans-Atlantic flights on a southern route. Aircraft can fly (at a fuel cost, admittedly) lower than the ash cloud. Then the tourists already in northern Europe could have been moved out before their situation became desperate.
Governments could have anticipated that passengers stuck longer than planned would need food, a place to sleep, access to their baggage or a change of clothes and could have appealed to local charities and citizens to help out as well as supplying some relief directly.
Instead, both governments and airlines let the situation develop until all flights were grounded, and continue to act as if passengers exist only to pay money...watching the BBC report last night, it's clear that airlines are still telling people to "stay here, check back in x hours" when "here" is a crowded noisy terminal with no place to lie down, no place to shower, to wash clothes, or to eat anything but expensive airport food. As usual, airlines have disclaimed all responsibility (though this problem was predictable from the moment that volcano first belched.) There's no evidence (from the BBC report anyway) that governments have done anything to relieve the situation for passengers.
The volcano's exact time of eruption could not be predicted, that's true. But an eruption "sometime" could be, as Iceland is known to have active volcanoes. Thus--with Iceland's location and the prevailing winds--the effect of a large ash-heavy eruption on major air routes could have been predicted. _Should_ have been predicted, with plans in place for minimizing the bad effects.