core-cli: backport more refactors from rewrite

Still a long way to go, but this introduces a few niceties for
debugging CLI failures:

+ The (extended) output of the last bin/doom command is now logged to
  ~/.emacs.d/.local/doom.log
+ If an error occurs, short backtraces are displayed whether or not you
  have debug mode on. The full backtrace is written to
  ~/.emacs.d/.local/doom.error.log.
+ bin/doom now aborts with a warning if:
  - The script itself or its parent directory is a symlink. It's fine if
    ~/.emacs.d is symlinked though.
  - Running bin/doom as root when your DOOMDIR isn't in /root/.
  - If you're sporting Emacs 26.1 (now handled in the elisp side rather
    than the /bin/sh shebang preamble).
+ If a 'doom sync' was aborted prematurely, you'll be warned that Doom
  was left in an inconsistent state and that you must run `doom sync`
  again.

May address #3746

core/autoload/output.el

@@ -254,8 +254,12 @@ DEST can be one or more of `standard-output', a buffer, a file"
                (insert-char out))
              (send-string-to-terminal (char-to-string out)))))
      (letf! (defun message (msg &rest args)
-              (princ (apply #'format msg args))
-              (terpri))
+              (with-current-buffer log-buffer
+                (print-group!
+                 (insert (doom--format (apply #'format msg args)) "\n")))
+              (if doom-debug-p
+                  (doom--print (doom--format (apply #'format msg args)))
+                (apply message msg args)))
        (unwind-protect
            ,(macroexp-progn body)
          (with-current-buffer log-buffer