Add RunScript backwards compatibility

Fixes regression introduced by #5563. Apparently, many add-ons are using `RunScript` to run other non-script addons. This 'feature' must go if addons are to be allowed to have 'script' as an addition extension points unambiguously. Adds backwards compatibility and warns about this instead. 

Also adds add similar warning to RunAddon: *log* when addon isn't a script/plugin and nothing will be executed.