patch 8.2.0522: several errors are not tested for

Problem:    Several errors are not tested for.
Solution:   Add tests. (Yegappan Lakshmanan, closes #5892)
diff --git a/src/testdir/test_normal.vim b/src/testdir/test_normal.vim
index 83a95f9..d8afeea 100644
--- a/src/testdir/test_normal.vim
+++ b/src/testdir/test_normal.vim
@@ -364,6 +364,7 @@
   " clean up
   unmap <buffer> ,,
   set opfunc=
+  call assert_fails('normal Vg@', 'E774:')
   bw!
   unlet! g:opt
 endfunc