[BUG] If an archive is already gone in Azure, SHIELD will never stop trying to purge it

[thomasmitchell]

Describe the bug
If an archive goes missing from the Azure storage out-of-band with SHIELD (or possibly, if SHIELD was restored back to a point when the archive existed), then SHIELD will, understandably, attempt to purge it. However, the purge will fail with a not found error. In response to the purge failing, SHIELD will schedule another attempt at some point. This cycle can't break.

To Reproduce
Steps to reproduce the behavior:

1. Make an archive in azure
2. Delete it out of band
3. Watch what happens when it actually expires

Expected behavior
The purge task should succeed if the archive is already gone

Screenshots
If applicable, add screenshots to help explain your problem.

SHIELD versions (please complete the following information):

• SHIELD Core: 8.5.0
• SHIELD CLI: n/a

Browser version(s) (please complete if reporting a web UI bug):
n/a

Additional context
Add any other context about the problem here.

This could be solved if we can determine, in the Azure plugin, if an archive is actually gone, vs the plugin just not being authorized to see it anymore. I'm not sure if Azure returns 404s for unauthorized/forbidden when attempting to act on blobs.