patch 8.1.1863: confusing error when using a builtin function as method

Problem:    Confusing error when using a builtin function as method while it
            does not support that.
Solution:   Add a specific error message.
diff --git a/src/version.c b/src/version.c
index ba37a41..3adfdd0 100644
--- a/src/version.c
+++ b/src/version.c
@@ -770,6 +770,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1863,
+/**/
     1862,
 /**/
     1861,