Stop parsing on entities boundaries errors

For https://bugzilla.gnome.org/show_bug.cgi?id=744980

There are times, like on unterminated entities that it's preferable to
stop parsing, even if that means less error reporting. Entities are
feeding the parser on further processing, and if they are ill defined
then it's possible to get the parser to bug. Also do the same on
Conditional Sections if the input is broken, as the structure of
the document can't be guessed.