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_regexp_latin.vim b/src/testdir/test_regexp_latin.vim
index 15ab84c..ac2b15c 100644
--- a/src/testdir/test_regexp_latin.vim
+++ b/src/testdir/test_regexp_latin.vim
@@ -83,7 +83,7 @@
   set re=2
   call assert_fails('/a**', 'E871:')
   call assert_fails('/a*\+', 'E871:')
-  call assert_fails('/a\{a}', 'E870:')
+  call assert_fails('/a\{a}', 'E554:')
   set re=0
 endfunc