Delete dangling downloads if transform fails

[ponyisi]

Optimistic downloads of the results of a transformation might result in local copies of files existing for a transformation that later fails. Do a cleanup of any downloaded files should a transformation fail.

[gordonwatts]

Moved to 3.1 so we can layout a complete contract with the user for the case of failed or incomplete transforms (e.g. can user get and on partial transforms, what happens when user tries to run again, can user specify behavior, etc.).