Don't write an error if a PDF is encrypted and the indexer cannot open it

If the document is encrypted, it's not an error.

2013-09-23 08:07:16,109 [schedulerFactoryBean_Worker-3] ERROR EPAbstractHandler - OLAT::ERROR ^% N1-E3 ^% org.olat.portfolio
Could not get content of file Animalia.pdf (file-artefact 520355849)
>>>stack of 1.cause::org.olat.search.service.document.file.DocumentAccessException: PDF is encrypted. Can not read content file=Animalia.pdf
-> at org.olat.search.service.document.file.pdf.PdfBoxExtractor.extractTextFromPdf(PdfBoxExtractor.java:74)
at org.olat.search.service.document.file.pdf.PdfBoxExtractor.extract(PdfBoxExtractor.java:50)
at org.olat.search.service.document.file.PdfDocument.readContent(PdfDocument.java:107)
at org.olat.search.service.document.file.FileDocument.init(FileDocument.java:70)
at org.olat.search.service.document.file.PdfDocument.createDocument(PdfDocument.java:71)
at org.olat.search.service.document.file.FileDocumentFactory.createDocument(FileDocumentFactory.java:98)
at org.olat.portfolio.model.artefacts.FileArtefactHandler[...]