Feedparser: Store libXML fatal error messages in an array

All fatal libXML errors that occurred during parsing are stored in an array. There is also an access function for that array so it can be used by the caller, for example in `update_rss_feed` or some plugin.

This commit also fixes error reporting for "Unknown/unsupported type of feeds" which I broke by setting `$this->error` on line 66. I'm sorry again… :flushed: