Handle the correct exception when formatting extremely high amounts of bytes

Found in production. Yes really. Due to some borked LXC integration, my
snikket host reports

```
MemTotal:       9007199254740991 kB
MemFree:        9007199254690591 kB
MemAvailable:   9007199254690591 kB
```

That is more than 1024 TiB, so it tries to go further up in the scale,
which then causes a Guru Meditation because of the uncaught IndexError.

snikket_web/infra.py

@@ -60,7 +60,7 @@ def format_bytes(n: float) -> str:
     try:
         unit = BYTE_UNIT_SCALE_MAP[scale]
         factor = 1024**scale
-    except ValueError:
+    except IndexError:
         unit = "TiB"
         factor = 1024**4
     if factor > 1: