improve error message for call depth limit

Reported by @axtens in Slack:

> Crashed the nim runner with the following code in Reverse String

>```Nim
>import sequtils, strutils
>
>var seq_reverse: proc( str: seq[char]): seq[char]
>
>seq_reverse = proc( str: seq[char]): seq[char] =
> if str.len == 1:
> str[0..0]
> else:
> var slice = str
> slice.delete(0,0)
> #let revved = reverse(slice)
> concat(seq_reverse(slice), str[0..0])
>
>proc reverse*(str: string): string =
> cast[string](seq_reverse(toSeq(str)))
>
>echo reverse("dog")
>```

> Worked fine in Nim Playground, see https://play.nim-lang.org/#ix=3s0y

---

The error:

```
parsejson.nim(522) raiseParseErr
Error: unhandled exception: /mnt/exercism-iteration/results.json(1, 0) Error: { expected [JsonParsingError]
```

The website produces the message:

> An error occurred while running your tests. This might mean that there was an issue in our infrastructure, or it might mean that you have something in your code that's causing our systems to break.
>
> Please check your code, and if nothing seems to be wrong, try running the tests again.