Feedparser: Change error handling a bit

I had some trouble with [this feed](http://www.jensweinreich.de/feed/). It turned out that it contains some unicode chars that are not valid in XML, but libxml continued parsing for a while, so the last error wasn't a 9 and the error handling could not be triggered.
Cycling through the libxml errors makes error 9 detection possible in this case. I also changed the error reporting so that only fatal libxml errors are reported. Currently, only the first fatal error will be displayed.