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_spell.vim b/src/testdir/test_spell.vim
index fbd5c1a..c2de5ed 100644
--- a/src/testdir/test_spell.vim
+++ b/src/testdir/test_spell.vim
@@ -334,7 +334,7 @@
     return [[{}, {}]]
   endfunc
   set spellsuggest=expr:MySuggest3()
-  call assert_fails("call spellsuggest('baord')", 'E728:')
+  call assert_fails("call spellsuggest('baord')", 'E731:')
 
   set nospell spellsuggest&
   delfunc MySuggest