vm: don't print out arrow message for custom error

In `AppendExceptionLine()`, which is used both by the `vm`
module and the uncaught exception handler, don’t print anything
to stderr when called from the `vm` module, even if the
thrown object is not a native error instance.

Fixes: https://github.com/nodejs/node/issues/7397
PR-URL: https://github.com/nodejs/node/pull/7398


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>