Work around infinite recursion in scanner on Chrome.

This is a quick fix for
<https://code.google.com/p/google-caja/issues/detail?id=1848>
which should be replaced with something more principled once we figure
out what that is.

* Do not PLAIN_CALL any getter with a property named "stack".
* Add a limit for depth of traversal. (This will cause the test to fail
  rather than run out of time/memory, in the event of any future similar
  problems.)

Supporting changes:
* Move all definitions in test-scan-guest relating to Error to one place
  and use new Ref definition style.

@r5578

[felix8a, 2013-08-24 03:32:12]

lgtm