JS driver - Expose the errors constructors

[neumino]

When checking the type of the error, one way to do it is through the attribute name.

We can also export the constructor so people can check if an error is an instanceof constructors.

It's pretty easy to do, and is just a few lines of code.

[neumino]

Up in review 1177 assigned to @AtnNn
Branch michel_1926_js_errors

> r.error("foo").run(c, function(e, result) { console.log(e instanceof r.Error.RqlRuntimeError) })
undefined
> true

[neumino]

Review complete.

Merged in next as 9198850
Cherry picked in v1.11.x as 8ff6dbe

[dminkovsky]

Gracias!