cubicweb #2267081 Improve error message when ERQLExpression fails on "Constant" RQL variable. [validation pending]

the current message "" is not very helpful.

sugguested output: 'No read acces on "X" with eid 42.'

prioritynormal
typeenhancement
done in3.14.7
load1.000
load left0.000
closed by#baa3ea5eac2b querier: Improve error message when ERQLExpression fails on some entity (closes #2267081)