Improve Error Message for Missing JSON Keys when Loading into C++ Variable

[pwintz]

I think it would be helpful to improve the error messages when trying to read a key is not present in a nlohmann::json object. In particular, I have code like this:

  std::ifstream config_json_file(config_file_path);
  json json_data = json::parse(config_json_file);
  config_json_file.close();
  double tau = json_data["tau"];

If "tau" is not present in the JSON data, however, the following error is printed:

terminate called after throwing an instance of 'nlohmann::json_abi_v3_11_3::detail::type_error'
   what():  [json.exception.type_error.302] type must be number, but is null

This error doesn't give very helpful info for figuring out the problem. It would be helpful to print something like

The key "tau" is not found. Expected it to be a number.

[nlohmann]

You may want to check out https://json.nlohmann.me/api/macros/json_diagnostics/