Set error=false chunk default to fail builds on uncaught errors

[ChrisRackauckas-Claude]

Summary

• Adds Weave.set_chunk_defaults!(:error => false) at the start of weave_file, so benchmark builds now fail if any code chunk throws an uncaught error. Previously, Weave's default (error=true) would silently capture errors and include ugly stacktraces in published output.
• Adds a contributor guideline section to the README explaining how to properly handle error demonstrations in benchmarks.

Motivation

Benchmarks should fail loudly when code errors unexpectedly, rather than silently producing output with stacktraces. This makes CI more useful for catching real regressions. Chunks that intentionally demonstrate errors (e.g., showing why a method was omitted) need to explicitly opt in via error=true or use try/catch blocks.

This pairs with PR #1511 which already wrapped existing error demonstrations in try/catch.

Changes

src/SciMLBenchmarks.jl: Added Weave.set_chunk_defaults!(:error => false) as the first line of weave_file. This sets the default for all chunks in every benchmark — individual chunks can still override with error=true.

README.md: Added a "Handling Method Omissions and Errors in Benchmarks" section under Contributing with:

• Guidance that showing why methods fail is valuable
• Two approaches for capturing errors: try/catch (preferred for friendly messages) and error=true chunk option
• Examples of both approaches
• Warning that raw uncaught errors will fail CI

Test plan

• Verify existing benchmarks still build (they should, since Wrap error-demonstration sections in try/catch for clean builds #1511 already wrapped error demonstrations)
• Confirm that a benchmark with a genuinely broken code chunk now fails the build instead of silently producing error output

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