patch 8.2.0125: :mode no longer works for any system

Problem:    :mode no longer works for any system.
Solution:   Always give an error message.
diff --git a/src/ex_docmd.c b/src/ex_docmd.c
index e11ad08..78aee97 100644
--- a/src/ex_docmd.c
+++ b/src/ex_docmd.c
@@ -5927,7 +5927,7 @@
     if (*eap->arg == NUL)
 	shell_resized();
     else
-	mch_screenmode(eap->arg);
+	emsg(_(e_screenmode));
 }
 
 /*