Handle case where template file doesn't exist.

If a template file is a dangling symbolic link, it used to raise a
"TypeError: 'NoneType' object is not iterable".  Replace that useless
error with a NoTemplate exception reporting the problem file.

Author: rouilj

roundup/cgi/templating.py

@@ -298,6 +298,7 @@ def _find(self, name):
                 return # will raise invalid template
             if os.path.exists(src):
                 return (src, f)
+        raise NoTemplate("Unable to load template: %s"%name)
 
     def check(self, name):
         return bool(self._find(name))