Retrieve and display tracebacks for failed 'Save code now' tasks

In order to get some feedback about why a 'Save code now' task has failed, it would be great if we could retrieve the associated traceback, store it in the swh-web database and then display it in the 'Save code now' tasks list UI (maybe using a popover).

One solution could be to query our elasticsearch cluster, storing the logs of all origin loading tasks, to get those information.

Migrated from T1976 (view on Phabricator)

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