vim-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 vim/vim#10756)

c963ec31a0

Co-authored-by: Bram Moolenaar <Bram@vim.org>

test/old/testdir/test_cmdwin.vim

@@ -0,0 +1,17 @@
+" Tests for editing the command line.
+
+source check.vim
+source screendump.vim
+
+
+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