Guard against the stack property of error object being a string.

This is a workaround for a regression in v8:
https://code.google.com/p/v8/issues/detail?id=2559

In addition, Firefox provides the stack property as a string without
calling Error.prepareStackTrace. (It is possible to run gjstest in
Firefox via the browser runner.)
Merge pull request #5 from elau8465/stack_string

Guard against the stack property of error object being a string