Volume risk in an electricity market
where would the overproduced electricity go, if the consumer has
  estimated his consumption poorly?


It doesn't go anywhere - if the consumer under-consumes compared to the original prediction then, that under-consumption is physically less-current being taken and less-power being supplied by the generator. 

Electricity isn't like a delivery van setting off to deliver bread or cakes - it's much more instant than that and if a load takes less current or power then it's pretty much instantly reflected in the generator producing less power and the oil/coal/energy being used by the turbine and generator being less.

There is a financial deficit only and that gets resolved by credit arrangements (my guess).