Throw better error message when redefining a property

[Robdel12]

It's not obvious that there are default properties on an interactor like isVisible. If you set a property on an interactor with the same key you'll get an error like:

Uncaught Error: cannot redefine existing property "isVisible"

In an ideal world we should throw an error saying something like "Interactor already has a property named isVisible, please choose a new name"

Or something that says that interactor provides this. I spent ~5 mins looking for a duplicate until I realized what was going on.

[wwilsman]

Can this be closed with the updated property name restrictions? Or should we still address the new "reserved property name" error?

[wwilsman]

Going to assume this can be closed with the new error

[Robdel12]

Yea that makes sense