test storage server behaviour for bad container files

[daira]

The behaviour of a storage server on shares with a corrupt container (bad header or inconsistent length) is insufficiently tested. We should simulate various kinds of container corruption (also on a subset of shares in a shareset that has more than one share), to test that the correct errors are raised.

See for example ​this bug, which showed that raising CorruptStoredShareError on an immutable share with an inconsistent length was untested.

Note that the code and the intended behaviour (see #1566) has changed on the cloud branch so that is what we should test. The cloud backend has additional failure modes due to missing or inconsistent chunk objects.