KCL: Nicer error messages

[adamchalmers]

Fixes #10855. As Jon pointed out, there were many places that the KCL was including Rust debug strings instead of printing nice, human-friendly error messages. I've gone through and found a bunch of these, replacing the debug strings with better messages.

Here's one such example. Before:

After:

[vercel]

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Project	Deployment	Actions	Updated (UTC)
modeling-app	Ready	Preview, Comment	Apr 10, 2026 3:13am

[codspeed-hq]

Merging this PR will not alter performance

✅ 167 untouched benchmarks
🆕 2 new benchmarks
⏩ 93 skipped benchmarks¹

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
🆕	Simulation	parse_big_sketch_block	N/A	120.7 ms	N/A
🆕	Simulation	bench_edit_segment	N/A	709.5 ms	N/A

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_{Comparing achalmers/fix-error-msgs (dda53e3) with main (007cc86)²}

Footnotes

1. 93 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

2. No successful run was found on main (b051d9b) during the generation of this report, so 007cc86 was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩