cubicweb #17256169 improve RQL error messages [done]

see messages from nicola spanti in ML archives on 2019-11-15

https://lists.cubicweb.org/pipermail/cubicweb-devel/2019-November/110566.html

priorityimportant
typeenhancement
done in<not specified>
load left0.000
closed by<not specified>