Findbugs: fix errored detected on 1.8

[mcRoot]

Findbugs detected a few potential bugs on release 1.8 that should be coped with:

[INFO] BugInstance size is 3
[INFO] Error size is 0
[INFO] Total bugs: 3
[INFO] Return value of org.jsoup.nodes.Element.appendElement(String) ignored, is this OK in docet.engine.PDFDocumentHandler.normaliseHTML(Document) [docet.engine.PDFDocumentHandler] At PDFDocumentHandler.java:[line 553] RV_RETURN_VALUE_IGNORED_INFERRED
[INFO] Unread field: docet.engine.PDFDocumentHandler$DocumentPart.layout [docet.engine.PDFDocumentHandler$DocumentPart] At PDFDocumentHandler.java:[line 1087] URF_UNREAD_FIELD
[INFO] Unused field: docet.engine.PDFDocumentHandler$DocumentPart.id [docet.engine.PDFDocumentHandler$DocumentPart] In PDFDocumentHandler.java UUF_UNUSED_FIELD