Errors in manifest do not include message or stack traces

[jnicklas]

The error handler attached by the lane runner cannot capture the stack trace of errors raised in the manifest, because they originate from a different origin than the lane runner and the error appears simply as a generic "Script error" with no stack information or message. This is due to security restrictions in the browser: a script should not be able to interfere with another script from a different origin by capturing its errors. To fix this, we would probably need to move the manifest onto the same origin. See #584. I have not been able to find any other way around this restriction, as far as I can see there is no way to enable this via headers or security policies or the like.