Added checking for an exception from Object#inspect in mirb

mirb fails to print results when Object#inspect throws an exception.

```bash
$ bin/mirb
mirb - Embeddable Interactive Ruby Shell

This is a very early version, please test and report errors.
Thanks :)

> 2 ** 1000
 =>
> aaa
RangeError: Too large expornent.
>
```

### fixed:
```bash
$ bin/mirb
mirb - Embeddable Interactive Ruby Shell

This is a very early version, please test and report errors.
Thanks :)

> 2 ** 1000
RangeError: Too large expornent.
>
```