Conversion: Ignore un-parseable links when trying to generate ToC from links, instead of aborting the conversion. Fixes #1383365 [Ipv6 error when converting PDF to MOBI](https://bugs.launchpad.net/calibre/+bug/1383365)
Fix #1383508 [Little typo](https://bugs.launchpad.net/calibre/+bug/1383508)