Add debug logging for unknown Notion errors

[bachya]

Proposed change

Per #76394, Notion sometimes gets an unknown error during coordinator update, and the message alone isn't enough to determine what it is. This PR adds the corresponding traceback as a DEBUG-level log so that we can investigate further.

I also removed a constant in the same file that wasn't used anywhere.

Type of change

• Dependency upgrade
• Bugfix (non-breaking change which fixes an issue)
• New integration (thank you!)
• New feature (which adds functionality to an existing integration)
• Deprecation (breaking change to happen in the future)
• Breaking change (fix/feature causing existing functionality to break)
• Code quality improvements to existing code or addition of tests

Additional information

• This PR fixes or closes issue: N/A
• This PR is related to issue: Unknown (and unhelpful) Notion error during update #76394
• Link to documentation pull request:

Checklist

• The code change is tested and works locally.
• Local tests pass. Your PR cannot be merged unless tests pass
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• Tests have been added to verify that the new code works.

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