Flattening out the Error module, couple other comments on it

[jmagaram]

I've been looking at the Error module. It is hard to raise exceptions because each of the different types are exposed in sub-modules like Error.SyntaxError that contain a single function - raise. At a glance you can't tell what the different types of exceptions are and must navigate into the sub-module to do it. The Intellisense menu looks like this...

I much prefer the Js.Exn way of doing it.

I notice an Error.make function. Is there anything you can do with it before raising it? Because if not, just remove make and go straight to throwing. If we want the ability to add custom properties first, I'm not sure how you'd do that. Consider adding some of the get and set functions from Object or convert to object.

[zth]

Adding those shortcuts (raiseEvalError, raiseRangeError etc) is fine by me, if that helps with the ergonomics.

You can create an Error in JS without throwing it directly, and presumably that's what Error.make emulates. So we shouldn't remove that.