improved pagebreak error messages and fixed xmlns:nordic error

 * added validation for page numbers to opf-and-html schema, for cross-document pagebreak validation
 * updated pagebreak validation rules to contain more information useful for locating the error
 * epub3.validate now loads all html documents in spine order, rather than simply all HTML documents.
   * TODO: check if this has any consequence for validating the navigation document in case the navigation document is not in the spine
 * added xmlns:nordic and xmlns:epub to the `<html>` element in the single-HTML representation when converting from EPUB to HTML