Bug Fix - store_async_block

- Swift Driver: API Contract changed and failure case was not updated to reflect the new contract
--- updated failure case for the new contract so it doesn't break the up-level code when failure occurs
--- updated the failure test for the new contract
- Disk Driver: There was no possible way for it to fail even if it failed to write a block
--- Had to remove the contextual file open as it was prevent 100% test coverage, documented it in the code
--- Added a specific test case that is skipped for the swift driver as it only applies to the disk driver to test the disk driver's failure case
Bug Fixes

- Bug Fix (Cassandra): Only insert the reftime when we actually have a block in storage since we store the reftime with the block data
- Bug Fix (DiskStorageDriver): store_block must be able to fail
- Bug Fix (BlockStorageDrivers): store_block failure values must match the structure of the success values