cubicweb #2408354 sql crash on query like Any ATTR, COUNT(X) WHERE EXISTS(X related Y, Y attr ATTR) [open]

we should either raise BadRQL query telling that EXISTS can't be used or silently drop exists (note there are already similar cases where the 'related' is simply moved to the outer scope)

prioritynormal
typebug
done in<not specified>
load0.500
load left0.500
closed by<not specified>