docs: def check(self):
          "Checks EPUB integrity"
          config = self.load_config()
          if not check_dependency_epubcheck():
              sys.exit(error('Unavailable command.'))
          epub_file = u"%s.epub" % config['fileroot']
          epub_path = join(CWD, 'build', epub_file)
          print success("Starting to check %s..." % epub_file)
          epubcheck = u'epubcheck %s' % epub_path
          epubcheck = shell(epubcheck.encode())
          for line in epubcheck.errors():
              print error(line)
          for line in epubcheck.output():
              print line