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_excmd.vim b/src/testdir/test_excmd.vim
index 2c693e2..150e258 100644
--- a/src/testdir/test_excmd.vim
+++ b/src/testdir/test_excmd.vim
@@ -335,8 +335,8 @@
   call assert_fails('redir abc', 'E475:')
   call assert_fails('redir => 1abc', 'E474:')
   call assert_fails('redir => a b', 'E488:')
-  call assert_fails('redir => abc[1]', 'E474:')
-  let b=0zFF
+  call assert_fails('redir => abc[1]', 'E121:')
+  let b = 0zFF
   call assert_fails('redir =>> b', 'E734:')
   unlet b