在Middleware或者App\Exceptions\Handler里捕获
Symfony\Component\HttpKernel\Exception\NotFoundHttpException

如果是Middleware

use Symfony\Component\HttpKernel\Exception\NotFoundHttpException;

public function handle($request, Closure $next) {
    try {
        return $next($request);
    } catch (NotFoundHttpException $e) {
        return response()->json(['msg'=>'NotFound']);
    }
}

2017.04.04 更新

已知laravel5的默认Exceptions\Handler会优先匹配404异常，所以建议在Handler进行处理。

修改app/Exceptions/Handler.php的render方法如下

/**
 * Render an exception into an HTTP response.
 *
 * @param  \Illuminate\Http\Request  $request
 * @param  \Exception  $exception
 * @return \Illuminate\Http\Response
 */
public function render($request, Exception $exception)
{
    if (is_a($exception, \Symfony\Component\HttpKernel\Exception\NotFoundHttpException::class) && $request->expectsJson()) {
        return response()->json(['msg'=>'NotFound']);
    } else {
        return parent::render($request, $exception);
    }
}