Create general SceneJS.Exception class for standardised error events

Currently an "error" event observed with SceneJS.onEvent comes in two flavours - with or without the message on the exception:

SceneJS.onEvent("error", function(e) {
    if (e.exception.message) {
        alert("Error: " + e.exception.message);
    } else {
        alert("Error: " + e.exception);
    }
 });

This because the exception may or may not be a SceneJS exception object.

Solution

Create a SceneJS.Exception class and wrap non-SceneJS exceptions in it, so we can catch them like this:

SceneJS.onEvent("error", function(e) {
        alert("Error: " + e.exception.message);        
 });