Improve timeout error for background search

[Swiddis]

Description

Just improves the error message if a timeout happens during the background search operation.

Before:

There was internal problem at backend - [RuntimeException] - java.sql.SQLException: exception while executing query: Failed to fetch data from the index: the background task failed or interrupted.\n Inner error: null

After:

Error occurred in OpenSearch engine: java.lang.InterruptedException: Execution of the index search timed out. Increase the timeout via the `ppl.query.timeout` setting.

Related Issues

N/A

Check List

• New functionality includes testing.
• New functionality has been documented.
• New functionality has javadoc added.
• New functionality has a user manual doc added.
• New PPL command checklist all confirmed.
• API changes companion pull request created.
• Commits are signed per the DCO using --signoff or -s.
• Public documentation issue/PR created.

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✨ Finishing touches

🧪 Generate unit tests (beta)

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