patch 8.2.2837: various code lines not covered by tests

Problem:    Various code lines not covered by tests.
Solution:   Add test cases. (Dominique Pellé, closes #8178)
diff --git a/src/testdir/test_excmd.vim b/src/testdir/test_excmd.vim
index f830be8..2e137f0 100644
--- a/src/testdir/test_excmd.vim
+++ b/src/testdir/test_excmd.vim
@@ -591,4 +591,10 @@
   sandbox call Sandbox_tests()
 endfunc
 
+func Test_command_not_implemented_E319()
+  if !has('mzscheme')
+    call assert_fails('mzscheme', 'E319:')
+  endif
+endfunc
+
 " vim: shiftwidth=2 sts=2 expandtab