Fix repl erroring on valid code.

gnode repl does not handle certain valid JS:

```js
> function *test () {}; test().next().value
Error: Line 1: Unexpected token ;
```

node repl wraps the input code in parens first, sees if there was a
'SyntaxError', if so, it will try again without parens.

This patch 'translates' all regenerator errors into SyntaxErrors.

No easy way to test this, passing input over stdin does not exhibit the
problem.

Closes #10.