Decrease the log about missing content attachment in AbstractSolrMetadataExtractor from error to warn

A race condition might exist when deleting an attachment leading for the document to still have the attachment while the content is already deleted: if such case happens during indexing, we obtain a log error while it's actually not a true error.
We mainly have those during the tests which is annoying.

This should be fixed with a better handling of this kind of problem directly in the APIs of XWikiDocument, not in the indexer. So to not increase complexity of the indexer, we can lower the log to a warning instead of an error.