[tzwenn]

Instead of throwing an error, I decided to use sys.exit(). Then the plugin did not fail but exits normally.

Spoiler:

#!/usr/bin/env  python
# -*- coding: utf-8 -*-

__license__   = 'GPL v3'
__copyright__ = '2012, Sven Dziadek sven . dziadek at gmx . de'
__docformat__ = 'restructuredtext de'

GET_MOBI=False
GET_PDF=True

'''
https://www.jungewelt.de/abo/onlineabo.php
'''
import os, urllib2, zipfile, sys
from calibre.web.feeds.news import BasicNewsRecipe
from calibre.ptempfile import PersistentTemporaryFile
from calibre.ptempfile import PersistentTemporaryDirectory
from calibre.ptempfile import PersistentTemporaryFile
from urlparse import urlparse


class jungeWeltOnlineAbo(BasicNewsRecipe):

    title = u'junge Welt Onlineabo'
    description = u'Das ePub Onlineabo der jungen Welt'
    language = 'de'
    lang = 'de-DE'

    __author__ = 'Sven Dziadek'
    needs_subscription = True

    conversion_options = {
        'no_default_epub_cover' : True
    }


    # based on achims' code on https://www.mobileread.com/forums/showpost.php?p=1816751&postcount=16
    def build_index(self):
        browser = self.get_browser()
        

        # new login process
        # must be done here so that browser is already at a right website
        response = browser.open('https://www.jungewelt.de/loginFailed.php')
        browser.select_form(nr=1)
        browser.form['username'] = self.username
        browser.form['password'] = self.password
        browser.submit()
        # now find the correct file, we will still use the ePub file
        epublink = browser.find_link(text_regex=re.compile('.*Downloads*'))
        response = browser.follow_link(epublink)
        epublink = browser.find_link(text_regex=re.compile('.*ePub-Datei*'))
        response = browser.follow_link(epublink)


        # Cheat calibre's recipe method, as in post from Starsom17
        self.report_progress(0,_('downloading epub'))

        dir = PersistentTemporaryDirectory()
        epub_file = PersistentTemporaryFile(suffix='.epub',dir=dir)
        epub_file.write(response.read())
        epub_file.close()
        zfile = zipfile.ZipFile(epub_file.name, 'r')
        self.report_progress(0.1,_('extracting epub'))
        zfile.extractall(self.output_dir)
        epub_file.close()
        index = os.path.join(self.output_dir, 'content.opf')
        self.report_progress(0.2,_('epub downloaded and extracted'))


        #
        # Now, download the remaining files
        #
        if (GET_MOBI):
           self.report_progress(0.3,_('downloading mobi'))
           mobi_file = PersistentTemporaryFile(suffix='.mobi',dir=dir)
           browser.back()
           response = browser.follow_link(mobilink)
           mobi_file.write(response.read())
           mobi_file.close()

        if (GET_PDF):
           self.report_progress(0.4,_('downloading pdf'))
           pdf_file = PersistentTemporaryFile(suffix='.pdf',dir=dir)
           browser.back()
           pdflink = browser.find_link(text_regex=re.compile('.*PDF-Datei*'))
           response = browser.follow_link(pdflink)
           pdf_file.write(response.read())
           pdf_file.close()
           
        # Get all formats into Calibre's database as one single book entry
        self.report_progress(0.6,_('Adding files to Calibre db'))
        cmd = "calibredb add -1 " + dir
        os.system(cmd)
        sys.exit(0)

        return index

Unfortunately something or someone adds another news ebook with the name "XXXXXX recipe out". The x are always different letters. The epub in it is invalid as it is empty (0byte).

Maybe it is easier to change that?