Treat StoreUnavailable as DataSourceNotFound
Treat StoreUnavailable as DataSourceNotFound

When StoreUnavailable errors occur while loading the RDB object from
the S3 store, treat them as DataSourceNotFound errors instead. Improve
the base class of DataSourceNotFound (it's an OSError, just like
ConnectionError). Slight refactor and shortening of variable names in
anticipation of next commit.

ska-sa/katdal