fix(api/errcode): extract enclosed error from a storage driver catch-all error

Whenever an unexpected error happens, the storage driver base client will wrap it in a catch-all error - storagedriver.Error (here). Because of it, if we get one of these errors we must extract the enclosed error before trying to identify the best HTTP status for it, otherwise, this will always lead to a 500 Internal Server Error.

Related to #344 (closed)