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/popupwin.c b/src/popupwin.c
index 99ef290..ea1a1e1 100644
--- a/src/popupwin.c
+++ b/src/popupwin.c
@@ -1336,7 +1336,7 @@
 }
 
     int
-not_in_popup_window()
+error_if_popup_window()
 {
     if (bt_popup(curwin->w_buffer))
     {