fetch: update fetch_cadc_file to work with previews

The addition of the ".sdf" is now optional.

Also raise fetching errors as JSAProcError.  (Users of this
function should not have to know that it is using the requests
module internally and therefore that it might raise
requests.exceptions.HTTPError.)
CADC: add routine to fetch previews from CADC

This takes a list of all of the output files for a job
and fetches those where the filename indicates that
the file is a preview.
State machine: add downloading of CADC previews