patch 8.2.0426: some errors were not tested for

Problem:    Some errors were not tested for.
Solution:   Add tests. (Dominique Pelle, closes #5824)
diff --git a/src/testdir/test_terminal.vim b/src/testdir/test_terminal.vim
index c922bea..f83ca51 100644
--- a/src/testdir/test_terminal.vim
+++ b/src/testdir/test_terminal.vim
@@ -2330,6 +2330,10 @@
   unlet! g:called_arg
 endfunc
 
+func Test_terminal_invalid_arg()
+  call assert_fails('terminal ++xyz', 'E181:')
+endfunc
+
 func Test_terminal_in_popup()
   CheckRunVimInTerminal