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_popup.vim b/src/testdir/test_popup.vim
index c7229fc..381a2c1 100644
--- a/src/testdir/test_popup.vim
+++ b/src/testdir/test_popup.vim
@@ -367,7 +367,7 @@
   setlocal completefunc=DummyCompleteTwo
   call setline(1, 'two')
   /^two
-  call assert_fails('call feedkeys("A\<C-X>\<C-U>\<C-N>\<Esc>", "x")', 'E764:')
+  call assert_fails('call feedkeys("A\<C-X>\<C-U>\<C-N>\<Esc>", "x")', 'E839:')
   call assert_notequal(winid, win_getid())
   q!
   call assert_equal(winid, win_getid())