Simplify error handling in mkdir_p()

generate-darwin-source-and-headers.py

@@ -91,9 +91,7 @@ def mkdir_p(path):
     try:
         os.makedirs(path)
     except OSError as exc:  # Python >2.5
-        if exc.errno == errno.EEXIST:
+        if exc.errno != errno.EEXIST:
-            pass
-        else:
             raise