Do you have other widgets on the same form? I've found often that one widget's errors can cause issues loading others, and thus, the error could not even be related to RichText.
Otherwise, if it truly having an issue loading that widget, it could not only be that the file does not exists. Inside that file, it has to identify itself using dojo.provide() with the correct path.
Also, curious as to why it is trying to load tinymcs.js when you said the file is called tiny_mce.js (note the underscore). Maybe the dojo require-provide paths are not in sync?