So that `mruby -e 'a'` does not print backtrace history, i.e.

```
% mruby -e 'a'
-e:1: undefined method 'a' (NoMethodError)
```

Instead of

```
% mruby -e 'a'
trace (most recent call last):
        [1] -e:1
-e:1: undefined method 'a' (NoMethodError)
```