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/files.el

@@ -146,8 +146,9 @@ If COOKIE doesn't exist, return NULL-VALUE."
     (insert-file-contents file nil 0 256)
     (if (re-search-forward (format "^;;;###%s " (regexp-quote (or cookie "if")))
                            nil t)
-        (let ((load-file-name file))
-          (eval (sexp-at-point) t))
+        (or (let ((load-file-name file))
+              (eval (sexp-at-point) t))
+            null-value)
       null-value)))
 
 ;;;###autoload