chore: Add secondary message for UnknownLoopBound error

[vezenovm]

Description

Problem*

Resolves #3687

Summary*

This simply adds a secondary message to clarify that using a slice length will lead to the Could not determine loop bound at compile-time error.

I was thinking of disabling this earlier during type check but we don't necessarily know that we will have a slice until codegen as arrays and slices are polymorphic with one another. I also thought about making a separate runtime error to check during loop unrolling but we are not necessarily going to still have the ArrayLen call codegen'd in SSA and will instead have the call's results for the loop condition, so this isn't a basic check and some more logic would be needed.

The best option for a specific error check seems to be during codegen_for before we codegen the end range. We can check whether we are making a slice ArrayLen call for the end range. These changes all felt a bit much as the error is quite clear except for this one error case and thus a secondary message felt sufficient.

If we would like the error to be more specific for the failure case then I can look at adding logic that triggers an error for a slice length loop bound.

An example of the error now:

Additional Context

Documentation*

Check one:

• No documentation needed.
• Documentation included in this PR.
• [Exceptional Case] Documentation to be submitted in a separate PR.

PR Checklist*

• I have tested the changes locally.
• I have formatted the changes with Prettier and/or cargo fmt on default settings.