getexp: fix handling of config object errors

This is a follow-up of the Python 3 port.
The `message` field is no longer supported in standard exceptions.

getexp

@@ -95,7 +95,7 @@ try:
                        dict([x.split('=', 1) for x in args.assigns]),
                        config_roots, getexp=args.getexp)
 except ExpConfigError as error:
-    die(error.message, status=2)
+    die(error, status=2)
 
 exp_dir_names = []
 if 'MODEL_DIR' in config:
@@ -115,7 +115,7 @@ elif args.key:
             else:
                 print(get_value(config, key))
     except KeyError as error:
-        die("invalid config name '{0}'".format(error.message))
+        die("invalid config name %s", error)
 elif args.verbose >= 2:
     config.indent_type = '  '
     lines = io.BytesIO()