Errors thrown from JavaScript client is strings and not errors

In JavaScript, you can throw any object, including strings, but to get stack traces one should throw Errors.

The generated JS code throws the error string directly, instead of wrapping it in new Error.

Node core only rejects with/throws Errors, never strings, and Thrift should follow the same standard.

https://nodejs.org/api/errors.html#errors_class_error

All errors generated by Node.js, including all System and JavaScript errors, will either be instances of, or inherit from, the Error class.

PR for the change here.