fix(emacs): don't explode if quick-lint-js reports a warning

If quick-lint-js prints a warning for whatever reason, the warning
output is mixed with emacs-lisp output. This causes our Flymake code
to parse the output incorrectly, leading to confusing error messages
for users.

Fix the output confusion by forwarding warning output (stderr) to
Flymake's log, leaving the emacs-lisp output (stdout) cleanly
parsable.

plugin/emacs/flymake-quicklintjs.el

@@ -62,13 +62,15 @@ REPORT-FN is Flymake's callback."
   (when (process-live-p flymake-quicklintjs--proc)
     (kill-process flymake-quicklintjs--proc))
   (let ((src-buf (current-buffer))
-        (output-buf (generate-new-buffer " *flymake-quicklintjs*")))
+        (stdout-buf (generate-new-buffer "*flymake-quicklintjs*"))
+        (stderr-buf (generate-new-buffer "*flymake-quicklintjs-stderr*")))
     (setq flymake-quicklintjs--proc
           (make-process
            :name "flymake-quicklintjs"
            :connection-type 'pipe
            :noquery t
-           :buffer output-buf
+           :buffer stdout-buf
+           :stderr stderr-buf
            :command `(,flymake-quicklintjs-program
                       ,@(let ((file (buffer-file-name)))
                           (if file
@@ -81,7 +83,11 @@ REPORT-FN is Flymake's callback."
              (unwind-protect
                  (when (and (eq 'exit (process-status p))
                             (eq p flymake-quicklintjs--proc))
-                   (with-current-buffer output-buf
+                   (with-current-buffer stderr-buf
+                     (flymake-log :warning "%S"
+                                  (buffer-substring-no-properties
+                                   (point-min) (point-max))))
+                   (with-current-buffer stdout-buf
                      (let ((diags (flymake-quicklintjs--make-diagnostics
                                   src-buf
                                   (car (read-from-string
@@ -96,7 +102,8 @@ REPORT-FN is Flymake's callback."
                                    (point-min) (progn (goto-char (point-min))
                                                       (line-end-position))))))))
                (unless (process-live-p p)
-                 (kill-buffer output-buf))))))
+                 (kill-buffer stdout-buf)
+                 (kill-buffer stderr-buf))))))
     (process-send-region flymake-quicklintjs--proc (point-min) (point-max))
     (process-send-eof flymake-quicklintjs--proc)))