Fix XSS attacks against SparqlServlet and ExplorerServlet

[danizen]

Address issue #29:

• Change SparqlServlet to always return content-type text/plain; charset=utf-8 on error.
• Change SparqlServlet to catch and return Spring exceptions pertaining to parameter conversion in a way that will not echo the parameter value, which may contain malicious code.

[danizen]

Please hold off on this, as our security team is also asking for fixes to the ExplorerServlet