`$call` should be a call or `NULL` in error objects

```
❯ processx::run("false")
Error in `processx::run("false")`:
! System command 'false' failed
---
Exit status: 1
Stderr: <empty>
---
Type .Last.error to see the more details.

❯ .Last.error$call
[1] "processx::run(\"false\")"
```

So we should use another field for the call string.