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/version.c b/src/version.c
index fcdf562..bf1a1f2 100644
--- a/src/version.c
+++ b/src/version.c
@@ -778,6 +778,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1555,
+/**/
     1554,
 /**/
     1553,