Log debug instead of error when package diff render fails

we probably don't want email noise about this..

tailbone/views/upgrades.py

@@ -231,7 +231,7 @@ class UpgradeView(MasterView):
                                class_='showing')
             return showing + diff.render_html()
         except:
-            log.exception("failed to render package diff for upgrade: {}".format(upgrade))
+            log.debug("failed to render package diff for upgrade: {}".format(upgrade), exc_info=True)
             return "(not available for this upgrade)"
 
     def changelog_link(self, project, url):