Tolerate `OperationalError` in viewlets.

An `OperationalError` can be raised when DB-migrations are not yet exectued.
We catch and log them in viewlets in order order to access `@@manage-upgrades`
to perform these upgrades.

Fixes #352.
Merge pull request #355 from 4teamwork/deif-tolerate-operational-errors

Tolerate `OperationalError` in actions and viewlets.