fix: update DocumentIsPausedError

[huayaoyue6]

Summary

The external_document_indexing_task imports an undeclared exception class named DocumentIsPausedException, which was renamed to DocumentIsPausedError on September 11, 2024. Therefore, the reference needs to be updated to the latest version.

Close #11403

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Checklist

Important

Please review the checklist below before submitting your pull request.

• This change requires a documentation update, included: Dify Document
• I understand that this PR may be closed in case there was no previous discussion or issues. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.
• I ran dev/reformat(backend) and cd web && npx lint-staged(frontend) to appease the lint gods

[huayaoyue6]

Fixes #11403