Add stacktrace to ParseError

Fixes #854. Also clean up the parse error test a little, while adding a .stack check.
Huh. Well, not sure I agree with that style of reporting. QUnit's own docs for .equals say "When it fails, both actual and expected values are displayed in the test result, in addition to a given message." Though I guess it doesn't say that it won't include that message in other places.

Well, I'll just remove that one, "Expected error message" sounds more like we're waiting for ANY message. A string comparison should be clear enough, the .throws is what really needs a message.