org-element-at-point: Demote error to warning when called from non-Org buffer

* lisp/org-element.el (org-element-at-point): Do not throw an error
when `org-element-at-point' is called from non-Org buffer.  A number
of third-party packages are incorrectly using `org-element-at-point'
this way, relying upon `org-element-at-point' working in certain
scenarios.  Throwing an error (as was done previously) made an urgent
call to fix this problem, even when `org-element-at-point' happened to
work in a particular scenario.  Now, we just display a warning to give
package author more time to address or work around this.

See https://github.com/alphapapa/org-web-tools/issues/61:

    @yantar92 I understand why this change was made, but perhaps it
    was a bit premature, given how much breakage it's causing
    downstream? It would seem appropriate to at least make it a
    warning for a full major version before making it an error, to
    give downstream devs and users a break.

lisp/org-element.el

@@ -8287,8 +8287,8 @@ This function may modify the match data."
     (setq epom (or epom (point)))
     (org-with-point-at epom
       (unless (derived-mode-p 'org-mode)
-        (error "`org-element-at-point' cannot be used in non-Org buffer %S (%s)"
-               (current-buffer) major-mode))
+        (warn "`org-element-at-point' cannot be used in non-Org buffer %S (%s)"
+              (current-buffer) major-mode))
       ;; Allow re-parsing when the command can benefit from it.
       (when (and cached-only
                  (memq this-command org-element--cache-non-modifying-commands))