patch 9.0.0064: confusing error when using "q:" in command line window

Problem:    Confusing error when using "q:" in command line window.
Solution:   Check for the situation and give a better error message.
            (closes #10756)
diff --git a/src/testdir/test_cmdwin.vim b/src/testdir/test_cmdwin.vim
index 45ecdd1..d641b29 100644
--- a/src/testdir/test_cmdwin.vim
+++ b/src/testdir/test_cmdwin.vim
@@ -356,5 +356,14 @@
   call assert_equal('', getcmdwintype())
 endfunc
 
+func Test_cant_open_cmdwin_in_cmdwin()
+  try
+    call feedkeys("q:q::q\<CR>", "x!")
+  catch
+    let caught = v:exception
+  endtry
+  call assert_match('E1292:', caught)
+endfunc
+
 
 " vim: shiftwidth=2 sts=2 expandtab