patch 8.2.1183: assert_fails() checks the last error message

Problem:    assert_fails() checks the last error message.
Solution:   Check the first error, it is more relevant.  Fix all the tests
            that rely on the old behavior.
diff --git a/src/testdir/test_window_cmd.vim b/src/testdir/test_window_cmd.vim
index aa619cd..5506dbe 100644
--- a/src/testdir/test_window_cmd.vim
+++ b/src/testdir/test_window_cmd.vim
@@ -569,7 +569,7 @@
   au * 0 vs xxx
   arg 0
   argadd
-  call assert_fails("all", "E249:")
+  call assert_fails("all", "E242:")
   au!
   bwipe xxx
   call assert_equal(&columns, winwidth(0))