Fix error format in Ljs_eval.get_prop.

There was a parenthesing issue in the `failwith` message oin `get_prop`.

Before the patch:

```
vlorentz@gommier:~/js/LambdaS5/tests(master)$ echo "{[#proto: 'Object']}['__proto__']" | ../obj/s5.d.byte -load init.heap /dev/stdin -continue-s5-eval

Uncaught error: "[interp] (/dev/stdin:1:0-33) get_prop on a non-object.  The expression was (get-prop "

Fatal error: exception Failure("Runtime error")
```


After the patch:

```
vlorentz@gommier:~/js/LambdaS5/tests(master)$ echo "{[#proto: 'Object']}['__proto__']" | ../obj/s5.d.byte -load init.heap /dev/stdin -continue-s5-eval

Uncaught error: "[interp] (/dev/stdin:1:0-33) get_prop on a non-object.  The expression was (get-prop "Object" __proto__)"

Fatal error: exception Failure("Runtime error")
```