Catalog initialization should not rely on Exists semantics

[johnataylor]

Adding to the catalog has the additional behavior of checking for the existence of a catalog in the container and if it is not present then it creates one. This code presumes the "Exists" call on the Azure Storage SDK is always reliable.

It appears it might be better to not rely on this call (internally it relies on seeing a 404).

For now we should remove the "smarts" of checking for an existing catalog - at least in the AppendOnlyCatalog. The logic is just there to make the classes easier to use. In the production scenario, however, we always now explicitly whether we are appending to a catalog or whether we are starting a new one. We would do better to make this distinction explicit.