Finished last tasks prior to testing.

- The external download-agent now throws a particular exception when the
  download-worker prematurely stops the request.
- We now translate the entry mtime to the correct TZ when touching files.
- We now raise an exception when the download-request's mtime isn't TZ-aware.
- _SyncedResourceHandle is now aware of the physical downloaded file-path.
- Eliminated a couple of other to-dos.
- _SyncedResourceHandle now opens the physical file, forwards undefined
  method calls to the internal file-resource, and wraps those calls in fault-
  checks.
- Adjusted how we forward errors from the download-worker back to watchers.
- We now properly cleanup the _SyncedResourceHandle object, post-yield.

Now ready for testing.