patch 8.1.1555: NOT_IN_POPUP_WINDOW is confusing

Problem:    NOT_IN_POPUP_WINDOW is confusing. (Andy Massimino)
Solution:   Rename to ERROR_IF_POPUP_WINDOW().
diff --git a/src/ex_cmds2.c b/src/ex_cmds2.c
index 3c73030..af7aa4b 100644
--- a/src/ex_cmds2.c
+++ b/src/ex_cmds2.c
@@ -1864,7 +1864,7 @@
     char_u	*p;
     int		old_arg_idx = curwin->w_arg_idx;
 
-    if (NOT_IN_POPUP_WINDOW)
+    if (ERROR_IF_POPUP_WINDOW)
 	return;
     if (argn < 0 || argn >= ARGCOUNT)
     {