Improve error message when in instance query $ is ambiguous.

[khanaffan]

Describe the bug
for query

select  $->COBIE? as cobie from bis.PhysicalElement pe
            join 
            (select 
                el.ecinstanceid as id, 
            from bis.element as el ) as aa on pe.TypeDefinition.id = aa.id 
            join bis.physicalPartition pp on pp.ecinstanceid = pe.model.id            
            group by cobie

we get error

Error: Expression 'ECClassId' in ECSQL statement is ambiguous.

Which basically mean that $->COBIE need to be <alias>.$->COBIE or pe.$->COBIE. This is because instance query need ECInstanceId and ECClassId and there are two ECClassId in scope. pp.ECClassId and pe.ECClassId

reported by @glenworrall