fix TypeError message

fix incorrect error messages like this:

```
mirb - Embeddable Interactive Ruby Shell

This is a very early version, please test and report errors.
Thanks :)

> 1 << nil
(mirb):1: can't convert  into Integer (TypeError)
> 1 << ""
(mirb):1: can't convert  into Integer (TypeError)
> 1 << []
(mirb):1: can't convert  into Integer (TypeError)
```