Handle all exceptions from download to ensure downloads are marked as failed

[newhoggy]

Handle all AWS exceptions by continuing and other exceptions by aborting.

Also some variables were named uploading and tUploading when they should say downloading and tDownloading.

These names used to be correct because once-up-a-time this was an upload queue, but for reasons*, it has been converted to a download queue, but this rename got missed.

• The reason being it's more fault-tolerant if cabal-cache tracks dependencies during download rather than upload because cabal-cache needs to keep the cabal store in a state where packages that are depended on exist. The download queue ensures packages are downloaded in an order whereby if cabal-cache is interrupted at any time, what has already been downloaded to the cabal store is correct.

[newhoggy]

Going to merge for testing purposes.