Suppress improper error from libxml

Some versions of libxml generate improper errors during parsing.  This causes otherwise well-formed XML handling to fail.  There are many, many bug reports about this issue.  Here are a random sample:

* https://groups.google.com/forum/?fromgroups=#!topic/silex-php/EVAHWTfibwo
* https://github.com/symfony/symfony/issues/7291#issuecomment-14563561
* http://www.php.net/manual/en/domdocument.validate.php#99818
* https://bugs.php.net/bug.php?id=46465
* http://trac.symfony-project.org/ticket/9999
* http://forum.pivotx.net/viewtopic.php?f=17&t=3278

This specific fix sets the Dom::loadXML() method up to ignore parsing validation explicitly, and then filters for the erroneous error message generated from the "bad" versions of libxml : "Validation failed: no DTD found !".