fix: return the correct error on io.CopyN failure

[joonas]

Description

The code was erroneously returning err instead of copyErr, which will be set to nil from the os.OpenFile above.

Instead, I believe the intent was to return copyErr if it is an unexpected error (i.e. not io.EOF).

Related Issue

Sorry, I didn't create an issue for this.

Checklist before merging

• Test, docs, adr added or updated as needed
• Contributor Guide Steps followed

[netlify]

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[brandtkeller]

Great catch!

[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/internal/split/split.go	0.00%	1 Missing ⚠️

Files with missing lines	Coverage Δ
src/internal/split/split.go	31.25% <0.00%> (ø)

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