Deprecation: #65465 - Deprecate errorLog in ReferenceIndex 

See forge#65465

Description 

The method \TYPO3\CMS\Core\Database\ReferenceIndex::error() and the according property \TYPO3\CMS\Core\Database\ReferenceIndex::errorLog have been marked as deprecated. It was not used and always empty.

Impact 

Calling ReferenceIndex::error() will throw a deprecation message. This function should not be used from outside the core.

Migration 

Extensions that used this property to log errors have to use their own errorLog.