Add proper parsing errors when failing to extract values

[rgrinberg]

This helped me debug #1736

[rgrinberg]

@Chris00 feel free to push the fix on top of this PR btw. I'm not entirely sure how the behavior of int_of_string changed since 4.02 so I'm not confident enough to fix this.

[rgrinberg]

Thanks @Chris00. I tested this out and it seems to work.

I'll let @emillon or @avsm give this an opportunity to review before merging.

[avsm]

Reads fine to me and build-tested on the ocaml-ctypes dune branch.

One observation about the code in stddune is that it uses the option return type rather than Error (Msg of string). In Mirage, we argue that errors should be self-descriptive, but the style in Stddune is that the caller of the option-returning function formats a proper error message. This seems pretty effective as well, but I wonder if gobbling the exception and discarding it in try_with is wise in the longer term... you could still have the "caller-formats-good-errors" discipline with Error return types as well.

[ghost]

In this case, the exception that is being discarded is Invalid_argument "bool_of_string" which would be of no-use to the user. We could possibly use (t, unit) result rather than t option, to make the failure clearer.

[rgrinberg]

Returning a proper message makes sense when are interested in the modes of failures. Doesn't seem to be the case here.