cubicweb-celerytask #16640842 Make celery-monitor "immune" to crash [validation pending]

for now, it's quite easy to crash the celery-monitor (eg. when a wf transition fails, which might be an expected behavior)

done in0.3.0
load left0.000
closed by#33bcf6380d51 [ccplugin] make celery-monitor more robust (closes #16640842)
patch[ccplugin] make celery-monitor more robust (closes #16640842) [applied][entities] improve workflow management of CeleryTasks (by celery-monitor) robustness (related to #16640842) [applied]