Update `validateJson` Function To Check For File Structure

[TimothyWillard]

Right now validateJson (which was previously validateFile inside the FileUploader component) just console.log`s a snippet and returns true. This leads to a vague error like:

TypeError: Cannot read properties of undefined (reading 'map')

whenever the user uploads an improperly formatted JSON file. In fact the entire beforeUpload hook in the FileUploader component could use another pass with an eye for not crashing the app whenever a bad input is uploaded.