Be resilient to file store errors

Currently if an unhandled error occurs while transferring files from a TemporaryFile to a released File the operation is aborted but not cleaned up. This means the temporary file remains in the files store but the database entry has been removed.

* [ ] Revert all DB modifications if any file store operation fails
* [ ] Ensure file store deletions are the final operation to occur in any request and exclude them from the reversion logic ^^
* [ ] Explore making file store changes transactional and also reversible (don't believe this is possible without storing everything in memory)

No files or tests are named. Start by tracing the TemporaryFile-to-released-File transfer and the associated database modifications, then inspect file-store operations and request cleanup ordering. Done means failed file-store operations revert database changes and deletions occur last without being reverted.

Written by the indexing model from the issue text.