Tolerate `OperationalError` in actions and viewlets.

An `OperationalError` can be raised when DB-migrations are not yet exectued.
We catch and log them in viewlets and actions in order order to access the `@@manage-upgrades` view
to perform these upgrades.

Fixes #352.
@phgross i would have to add this functionality to `OGDSService` or some of the `opengever.ogds.models` classes. I think this is out of scope for this PR and should be solved as part of another ticket and within a separate PR. I created #362 to keep track of this issue.