Make it more explicit when using 'contentdelete' on a property

When I used an item in hippoecm-extension.xml with a "contentdelete" pointing to a property, it took hours to figure out I had to use "contentpropdelete" instead. The info message does say "node" already but it's not in-the-face enough.

Let's at least give an error or warning, maybe introduce a "contentnodedelete", or have something else more explicit.