nightly-2024-06-23: fix: Replace panic in monomorphization with an error (#5305)

# Description

## Problem\*

## Summary\*

Replaces an `unwrap` and panic in the monomorphizer with an ICE issued
to the user instead. This isn't expected to be issued normally, but is
being issued currently when a `comptime let` variable is used in runtime
code since `comptime let` is still unimplemented in the evaluator.

## Additional Context

This does not fix the underlying `comptime let` error, only the
resulting panic.

## Documentation\*

Check one:
- [x] No documentation needed.
- [ ] Documentation included in this PR.
- [ ] **[For Experimental Features]** Documentation to be submitted in a
separate PR.

# PR Checklist\*

- [x] I have tested the changes locally.
- [x] I have formatted the changes with [Prettier](https://prettier.io/)
and/or `cargo fmt` on default settings.