knowledge_pad: ignore feedparser.NonXMLContentType

When server replies with a non XML content type feeparser attempt to
parse anyway, but set the bozo flag.

https://universal-feedparser.readthedocs.io/en/latest/#handling-incorrectly-declared-media-types

If parsing fail because the feed is not a valid RSS XML stream,
feedparser fail in other ways, so we can safely ignore this error.

This should repair recent failures with testFeedReaderGadget now that le
monde changed configuration on their website:

     $ curl -sD -  -o /dev/null https://www.lemonde.fr/rss/une.xml | grep Content-Type
     Content-Type: text/html;charset=UTF-8

/reviewed-on !1027