Hello @kartik,
You'll need to adjust your exception handler (app/Exceptions/Handler.php) to return the response you want.
This is a very basic example of what can be done.
public function render($request, Exception $e)
{
    $rendered = parent::render($request, $e);
    return response()->json([
        'error' => [
            'code' => $rendered->getStatusCode(),
            'message' => $e->getMessage(),
        ]
    ], $rendered->getStatusCode());
}