Replace `stopifnot` with descriptive error messages

I just got the following error message:

```
Error in stopifnot(is.character(description) && length(description) == :
argument "description" is missing, with no default
Calls:
```

That’s it. no context, nothing. It took quite a while to figure out that this was from a call to `opt` that was missing a description. Something like the following would be better:

```
Error in opt:
no description given for option "foo"
```

Or anything, really.

Furthermore, the lack of call stack is irritating too. This should really be fixed in `sys$run` but I have no idea how.

Start by tracing option validation around opt and the sys$run entry point, then identify where stopifnot produces the reported error. Done means missing option descriptions produce a descriptive error naming the option, and sys$run provides useful call-stack context as requested.

Written by the indexing model from the issue text.