patch 9.0.1249: cannot export an abstract class

Problem:    Cannot export an abstract class. (Ernie Rael)
Solution:   Add the EX_EXPORT flag to :abstract. (closes #11884)
diff --git a/src/vim9class.c b/src/vim9class.c
index 4845005..d64e35e 100644
--- a/src/vim9class.c
+++ b/src/vim9class.c
@@ -423,7 +423,7 @@
 	char *wrong_name = is_class ? "endinterface" : "endclass";
 	if (checkforcmd(&p, wrong_name, is_class ? 5 : 4))
 	{
-	    semsg(_(e_invalid_command_str), line);
+	    semsg(_(e_invalid_command_str_expected_str), line, end_name);
 	    break;
 	}