REPL should raise Exception immediately when it gets naturally invalid multi-line.

I'm using v5.0.0 on Windows.

Of course this is invalid as javascript:

``` javascript
a = 3.5e
-5;
console.log(a);
```

But REPL enters multi-line editting mode:

```
> a = 3.5e
... 
... 
... .break
```

We have no way to exit from the mode except `.break` (or Ctrl-C), and `.break` will make nothing.
