Mark tasks as failed if no ZIM file was created

[rgaudin]

Sort of a complement of #684 ; if the scraper completes with 0 yet we have not seen any ZIM files, something must be wrong and the task should be marked as failed.
In addition, a log message should be produced with the (recursive ? @kelson42 ?) content of /output so one can check whether something was created (temp ZIM not finished, incorrect extension).

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be now be reviewed manually. Thank you for your contributions.

[benoit74]

Removed from zimit2 project unfortunately

[benoit74]

The fact that we need to mark task as failed when no ZIMs are found has been tackled in #1045

Still need to log the content of output folder.