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_python2.vim b/src/testdir/test_python2.vim
index 42fadaa..8fa7340 100644
--- a/src/testdir/test_python2.vim
+++ b/src/testdir/test_python2.vim
@@ -73,7 +73,7 @@
 
   " Try modifying a buffer with 'nomodifiable' set
   set nomodifiable
-  call assert_fails('pydo toupper(line)', 'cannot save undo information')
+  call assert_fails('pydo toupper(line)', 'E21:')
   set modifiable
 
   " Invalid command