patch 8.0.0470: not enough testing for help commands

Problem:    Not enough testing for help commands.
Solution:   Add a few more help tests. (Dominique Pelle, closes #1565)
diff --git a/src/testdir/test_help.vim b/src/testdir/test_help.vim
index ca095d0..5a35b69 100644
--- a/src/testdir/test_help.vim
+++ b/src/testdir/test_help.vim
@@ -8,3 +8,8 @@
   help
   helpclose
 endfunc
+
+func Test_help_errors()
+  call assert_fails('help doesnotexist', 'E149:')
+  call assert_fails('help!', 'E478:')
+endfunc