Bug 1513951 - [mach] Fix error handling bug in config.py r=gbrown

Differential Revision: https://phabricator.services.mozilla.com/D21430

--HG--
extra : moz-landing-system : lando

python/mach/mach/config.py

@@ -380,7 +380,7 @@ class ConfigSettings(collections.Mapping):
             for k, v in settings.items():
                 if k in section:
                     raise ConfigException('Setting already registered: %s.%s' %
-                                          section_name, k)
+                                          (section_name, k))
 
                 section[k] = v