If any unexpected parameters are sent to the /people/search endpoint, it fails with a 400 Bad Request, and doesn't explain further in the body text. This is a problem, especially considering the API Explorer adds an empty __proto__ URL param, which is rejected by the API with the message above. Really, the API should ignore any irrelevant URL parameters, rather than just mysteriously dying. If there's a technical limitation around these for some reason, the response text should at least be more verbose.
Overall, we don't have any specialized error reporting through the API. If the request is incorrect, you are properly sent back a 400 bad request (or other respective response). We could set up the API to provide errors depending on typos or if the request contains irrelevant URL parameters, but we haven't prioritized this sort of project. As an alternative to the API Explorer, you could use your own tools to send requests through our "REST API" in NationBuilder. For example, you can use "curl" to send requests through terminal on a Mac (or command prompt on Windows).