Downloading XMM data should be allowed to retry at least once

[DavidT3]

If the connection is refused, then it should be allowed to attempt at least one more time to download the data.

[DavidT3]

There should also be a check for whether a zipped tar remains, because if so then I think something has gone wrong. This did happen as I tested re-processing all of XMM. So I ran a little script:

`import os
from shutil import rmtree

dirs = os.listdir('.')

tars = [f for f in dirs if 'tar.gz' in f]

for tar in tars:
obs_id = tar.split('.')[0]
if obs_id in dirs:
rmtree(obs_id)
os.remove(tar)`

To identify remaining zips, and remove any ObsID directory with the same name.

I think this should probably be a checking step in the DAXA downloading process.