Unicodify has different semantics to str

Originally my patch was written with str()'s behaviour in mind. If a value is None, it gets cast.
I did not test my patch well enough following @nsoranzo's request to replace with unicodify(). In that case a raw None gets passed to cgi.escape() causing a bug

lib/galaxy/tools/errors.py

@@ -209,7 +209,8 @@ def create_report( self, user, email='', message='', **kwd ):
 
         # Escape all of the content  for use in the HTML report
         for parameter in report_variables.keys():
-            report_variables[parameter] = cgi.escape(unicodify(report_variables[parameter]))
+            if report_variables[parameter] is not None:
+                report_variables[parameter] = cgi.escape(unicodify(report_variables[parameter]))
 
         self.html_report = string.Template( error_report_template_html ).safe_substitute( report_variables )