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/macros.h b/src/macros.h
index 0f5ab77..d685dea 100644
--- a/src/macros.h
+++ b/src/macros.h
@@ -340,8 +340,9 @@
 /* Wether a command index indicates a user command. */
 #define IS_USER_CMDIDX(idx) ((int)(idx) < 0)
 
+// Give an error in curwin is a popup window and evaluate to TRUE.
 #ifdef FEAT_TEXT_PROP
-# define NOT_IN_POPUP_WINDOW not_in_popup_window()
+# define ERROR_IF_POPUP_WINDOW error_if_popup_window()
 #else
-# define NOT_IN_POPUP_WINDOW 0
+# define ERROR_IF_POPUP_WINDOW 0
 #endif