Improve error messages (#305)

* small improvement

* validate Vue vnode

Also change Fragment to "template", *oops*.

* improve error messages in tests for Menu (Vue)

Also actually making sure that we have valid MenuItem components... By
default it renders a template, therefore `<MenuItem>Abc</MenuItem>` is
technically incorrect.

Author: RobinMalfait

packages/@headlessui-react/src/components/description/description.tsx

@@ -84,7 +84,7 @@ type DescriptionPropsWeControl = 'id'
 export function Description<TTag extends ElementType = typeof DEFAULT_DESCRIPTION_TAG>(
   props: Props<TTag, DescriptionRenderPropArg, DescriptionPropsWeControl>
 ) {
-  let { register, slot: slot } = useDescriptionContext()
+  let { register, slot } = useDescriptionContext()
   let id = `headlessui-description-${useId()}`
 
   useIsoMorphicEffect(() => register(id), [id, register])