Don't fail on already existsing directories

src/mio/node.py

@@ -255,7 +255,10 @@ class dir_node(target_node):
                 mio.exception.Report(e, self.decl, a)
 
     def mkdir(self):
+        if not os.path.isdir(self.target):
             os.makedirs(self.target, self.mode)
+            pass
+        pass
 
     def chmod(self):
         os.chmod(self.target, self.mode)