cubicweb-worker #7423207 provide reasonable info about the cause of worker death [done]

Known causes, such as instance shutdown (debug/service mode), task abortion, or byzantine failures, should be distinguishable easily.

prioritynormal
typeenhancement
done in4.0.0
load0.300
load left0.000
closed by#7ad4184229e9 Add an attribute on workers to record the probable cause of its end-of-life.
patchAdd an attribute on workers to record the probable cause of its end-of-life. [applied]