Walk the cause chain when dumping uncaught errors

[saghul]

Example:

var e = new Error('inner!');

throw new Error('outer!', { cause: e });

$ ./build/qjs t.js
Error: outer!
    at <anonymous> (t.js:3:11)

Caused by: Error: inner!
    at <anonymous> (t.js:1:12)

[bnoordhuis]

Maybe it should preserve rt->current_exception? Apart from that LGTM.

[saghul]

Maybe it should preserve rt->current_exception? Apart from that LGTM.

Goot point, pushed a fix that basically acts the way the previous code did with the stack property.