If calling the executor function throws an exception, the return value
of `vm.call()` will be an empty value, which we then passed as an
argument to the reject function, which is incorrect - what it actually
needs is the exception value. This stems from a misunderstanding of the
spec I had at the time of implementing this - in their case, the
exception value is part of the completion record returned by Call().
This error was previously masked as we would use a fallback
(`value_or(js_undefined())` for the empty value argument, but that was
removed in 57f7e6e.
Fixes#8447.
073071c634