Fix line wrapping and some raise messages spacing

* tiramisu/option.py (Multi.append): do not wrapp short line.
  (validate_requires_arg): Add missing space for multi line strings.

tiramisu/option.py

@@ -91,8 +91,7 @@ class Multi(list):
             if master == multi.opt._name:
                 if add_master:
                     multi._setvalue(value, who=settings.get_owner())
-            elif len(multi) == 0 \
-                 or len(multi) < len(self):
+            elif len(multi) == 0 or len(multi) < len(self):
                 multi._append_default()
 
     def _append_default(self):