Add __exception__ property only in enhanced java access

[rPraml]

I noticed that Rhino stores the current exception in a __exception__ variable.
This is not really standard and in my opinion it only makes sense when using LiveConnect (and even there I see no reason to enable this by default).

My suggestion is to enable this feature only via flag (FEATURE_ENHANCED_JAVA_ACCESS should fit) or remove it completely.

It was introduced with d0c1400 but no test or use case was described. And I also found only this post: https://groups.google.com/g/mozilla.dev.tech.js-engine.rhino/c/qNttXExynw4?pli=1

Instead of putting something in the current scope, you can use this more object oriented code to access the underlying exception

try {
   ... 
} catch (e) {
   e.rhinoException.printStackTrace();
   // or if it is a real java exception:
   e.javaException.printStackTrace();
}

So maybe anyone can tell a use case for this feature, if this is still needed?

[p-bakker]

This seems to me something very old, that has been surpassed by other features

I wouldn't mind deprecating this for removal and maybe already putting this behind some DEPRECATED_FOR_REMOVAL flag

Heck, I wouldn't mind just tossing it out completely right now 🤪

[rbri]

i do not use it in HtmlUnit so removal is fine for me and everything elso also.

[gbrail]

I think it makes sense and I'm going to merge this and put it behind the feature flag -- and given that it's not documented anywhere I'd be happy to remove it entirely later.