vcpkg-tool should provide better error message for JSON files with duplicated keys

[horenmar]

Today I ended up with a duplicated key in my vcpkg.json. vcpkg rightly errored on this, but the error message was almost entirely useless:

1>-- Running vcpkg install
1>CUSTOMBUILD : error : vcpkg has crashed; no additional details are available.
1>The source line is D:\a\_work\1\s\src\vcpkg\base\json.cpp(338)
1>

From the error message I can tell that the problem is in my json, aaaand... that I need to go to the sources to figure out why.

The solution is to add proper error message to the specific check (and ideally also go through other similar checks and improve their error messages).

---

For context, I had this JSON object in my project's dependencies:

    {
      "$comment": "We rely on opencv4, even though the include paths are opencv2/",
      "name": "opencv4",
      "default-features": false,
      "$comment": "Currently the package requires protobuf even though it does not depend on it properly. This feature brings in protobuf dependency.",
      "features": ["dnn"]
    },