chore: Fix raise for older tmux versions

Author: tony

libtmux/window.py

@@ -477,9 +477,8 @@ def split_window(
 
         # tmux < 1.7. This is added in 1.7.
         if pane.stderr:
-            raise exc.LibTmuxException(pane.stderr)
             if "pane too small" in pane.stderr:
-                pass
+                raise exc.LibTmuxException(pane.stderr)
 
             raise exc.LibTmuxException(pane.stderr, self._info, self.panes)
         else: