## Summary
This PR simplifies `IterationError` and `ContextManagerError` so that
they no longer "remember" what type it was that was (respectively) not
iterable or not valid as a context manager. Instead, the type that was
iterated over (or was used as a context manager) is passed back in when
calling the error struct's `report_diagnostic` method.
The motivations for this are:
- It significantly simplifies the code
- It reduces the size of these types on the stack
## Test Plan
`cargo test -p red_knot_python_semantic`
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