[Bug-apl] quad-ET doesn't follow SI

[David B. Lamkins]

quad-ET should always reflect the error that caused the top suspension
on the SI. When the SI is empty, quad-ET should yield 0 0.


      ⎕et
0 0
      foo
VALUE ERROR
      foo
      ^
      ⎕et
3 1
      1÷0
DOMAIN ERROR
      1÷0
      ^^
      ⎕et
5 4
      →
      ⎕et
5 4
^^ NOTE: should be 3 1
      →
      ⎕et
5 4
^^ NOTE: should be 0 0
      )si