Do not traceback when rendering a blurp (fixes #5092)

If DEBUG is True, report exception in result string.

src/cmsplugin_blurp/templatetags/blurp_tags.py

@@ -1,4 +1,8 @@
+import logging
+
 from django import template
+from django.conf import settings
+from django.utils.html import escape
 
 from classytags.arguments import Argument
 from classytags.core import Options, Tag
@@ -21,9 +25,19 @@ class RenderBlurp(Tag):
             return ''
         template = renderer.render_template()
         context = renderer.render(context)
-        if not hasattr(template, 'render'):
-            template = template.Template(template)
-        return template.render(context)
+        try:
+            if not hasattr(template, 'render'):
+                template = template.Template(template)
+            return template.render(context)
+        except Exception, e:
+            logging.getLogger(__name__).exception('error while rendering %s',
+                    renderer)
+            msg = ''
+            if settings.TEMPLATE_DEBUG:
+                msg += 'error while rendering %s: %s' % (renderer, e)
+                msg = '<pre>%s</pre>' % escape(msg)
+            return msg
+
 
 @register.tag
 class BlurpNode(Tag):