From the Eclipse Point of view the error is totally correct because JavaFX is coming from the extension classpath and is not available on ALL Java8 VMs (e.g. ibm!).
As outlined you can suppress those warnings by add access-rules or IMHO the best solution is to install e(fx)clipse which does this automatically for you and beside that even provides you tooling for JavaFX CSS and FXML.
You can grab an all in one package from http://efxclipse.bestsolution.at/install.html