DS-2275 XMLUI Error Page onclick errors

https://jira.duraspace.org/browse/DS-2275

This PR changes the javascript link to show the stack trace on an error page to be `<a onclick="javascript...` instead of an `<a href="javascript...`. To add the xref onclick, I had to add that to the core/elements.xsl. Chrome is smart enough to execute the javascript in an a href, but firefox instead left the page, and opened a javascript console page or something...

Also, my early diagnosis of what this issue was, was that I thought MathJax was causing the issue. Nope. But MathJax did try to render the stack trace, since it had enough dollar signs. I made MathJax not render the exception page.