This MR shows errors and exit in these conditions:
* multiple `<py-config>` or `<mpy-config>` found on the page
* both `<py-config>` and `<script type="py" config="file.toml">` found on main
* different `<script type="py" config="a.toml">` and `<script type="py" config="b.toml">` configs found on main