Arrange to give error when a SetOp member statement refers to a variable

Arrange to give error when a SetOp member statement refers to a variable
of the containing query (which really can only happen in a rule context).
Per example from Brandon Craig Rhodes.  Also, make the error message
more specific for the similar case with sub-select in FROM.  The revised
coding should be easier to adapt to SQL99's LATERAL(), when we get around
to supporting that.