DS-2275 Not render MathJax in error page exception
DS-2275 Fix browser support for error page link to show stack error

The link to show the stack trace, uses a javascript call, this should be
an onclick, not a href, otherwise a browser like firefox opens a js console
To make this work, I had to extend xref to have an onclick attribute.
Merge pull request #743 from LongsightGroup/DS-2275-mathjax-error

DS-2275 XMLUI Error Page onclick errors