Add specs for common failure cases

We don't have all the specs required to get the IMM point for 'Specs which cover common failure cases'

Specs for `Corvus.Tenancy.Storage.Azure.Blob` do include two failure cases, but there are issues including (but not necessarily limited to):

* you wouldn't know from the Scenario titles that these test failure cases
* no test for failure when trying to create a child under a non-existent parent
* no test for failure when trying to update a non-existent child
* no test for failure when trying to delete a non-existent child
* no test for failure when trying to get children of a non-existent child
* no test for whether errors caused by misconfiguration are intelligible

There are no specs for failure at all for the Cosmos, Gremlin, and Azure Storage tenanted storage providers.

Start by locating the existing specs for Corvus.Tenancy.Storage.Azure.Blob and the specs for the Cosmos, Gremlin, and Azure Storage tenanted providers. Review the current failure-case scenarios, then add coverage for the listed missing failures and for intelligible misconfiguration errors. Done means the common failure cases are covered with scenario titles that clearly identify failures.

Written by the indexing model from the issue text.