Rewrite org-download/attach systems

+ Cut down on unnecessary code & advice
+ Fix 'attach and 'download methods for org-download so that a) their
  links actually work and b) inline previews of their links show up.
+ Allow users to only set org-attach-id-dir (which will be mirrored to
  org-download-image-dir, unless the user has changed it).
+ Prevent a few edge cases where org-attach-id-dir or
  org-download-image-dir were blank.

modules/lang/org/autoload/org-attach.el

@@ -42,7 +42,7 @@ PATH (a string) can be an url, a local file path, or a base64 encoded datauri."
   (unless (eq major-mode 'org-mode)
     (user-error "Not in an org buffer"))
   (require 'org-download)
-  (condition-case ex
+  (condition-case-unless-debug e
       (let ((raw-uri (url-unhex-string path)))
         (cond ((string-match-p "^data:image/png;base64," path)
                (org-download-dnd-base64 path nil))
@@ -56,4 +56,4 @@ PATH (a string) can be an url, a local file path, or a base64 encoded datauri."
                  ;; insert the link
                  (org-download-insert-link raw-uri new-path)))))
     (error
-     (user-error "Failed to attach file: %s" (error-message-string ex)))))
+     (user-error "Failed to attach file: %s" (error-message-string e)))))