Update with correct param name.
Require Mercator 0.3.3.

The reason for this is that this is the first version where there are
guarantees that the rounding method used in all calculations is uniform,
both within a process and between various platforms. Previous versions
didn't guarantee this.
Handle missing entities.

This can happen if there's a transient entity, which is referenced but
not persisted.
Handle server errors better.

Report them and then continue on with trying to create the next entity.
Provide better import stats.