Improve reporting of extension config schema validation errors

[lf-]

In large extension configuration schemas, if a type error is present on a configuration key, an error message like the following is generated:

This requires the extension developer to go through each property and check it's the right type. I made it print out which one was bad:

This PR can be tested by updating the top level object of an extension package.json with this fragment:

{
    "contributes": {
        "configuration": {
            "type": "object",
            "title": "Rust Analyzer",
            "properties": {
                "rust-analyzer.thisPropShouldBeAnObjectButIsnt": false
           }
     }
}

[lf-]

(I think this is a preexisting broken build fwiw)