Allow checkdoc to be called in batch

* lisp/emacs-lisp/checkdoc.el (checkdoc-error): When `noninteractive'
  is non-nil, echo the error with `warn'.

How it can be used in -batch:

(with-current-buffer (find-file "checkdoc.el")
  (checkdoc-current-buffer t))

1 files changed, 6 insertions, 3 deletions

diff --git a/lisp/emacs-lisp/checkdoc.el b/lisp/emacs-lisp/checkdoc.el
index 777fed082d9..b3ef44a5a1c 100644
--- a/lisp/emacs-lisp/checkdoc.el
+++ b/lisp/emacs-lisp/checkdoc.el
@@ -2613,9 +2613,12 @@ function called to create the messages."
                      (count-lines (point-min) (or point (point-min))))
                     ": " msg)))
     (with-current-buffer (get-buffer checkdoc-diagnostic-buffer)
-      (goto-char (point-max))
-      (let ((inhibit-read-only t))
-        (apply #'insert text)))))
+      (let ((inhibit-read-only t)
+            (pt (point-max)))
+        (goto-char pt)
+        (apply #'insert text)
+        (when noninteractive
+          (warn (buffer-substring pt (point-max))))))))
 
 (defun checkdoc-show-diagnostics ()
   "Display the checkdoc diagnostic buffer in a temporary window."