fix: error message for discard

[ianlewis]

Description:

Fix the error message when discarding runes to use the error message from the discard operation.

Related Issues:

Fixes #182

Checklist:

• Review the CONTRIBUTING.md documentation.
• Add a reference to a related issue in the repository.
• Add a description of the changes proposed in the pull request.
• Add unit tests if applicable.
• Update documentation if applicable.
• Add a note in the CHANGELOG.md if applicable.

[Copilot]

Pull request overview

This PR fixes an error handling bug where the wrong error variable was being used when reporting discard operation failures, and cleans up error handling logic by renaming variables for clarity.

Changes:

• Corrected error variable reference in discard error message from err to dErr
• Renamed err to peekErr throughout the function to distinguish peek errors from discard errors
• Removed redundant error setting for EOF condition

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