org-element-at-point: Throw an error when not in org-mode

* lisp/org-element.el (org-element-at-point): Explicitly throw an
error when not in org-mode buffer.  The parser expects certain
buffer-local constants and may fail otherwise.  It is better to
indicate the problem explicitly instead of dealing with cryptic
generic errors.
(org-element-plain-list-interpreter):
* testing/lisp/test-ox.el (test-org-export/expand-include/links): Make
sure that we never invoke parser in non-Org buffers.

lisp/org-element.el

@@ -1780,6 +1780,7 @@ Assume point is at the beginning of the list."
   "Interpret plain-list element as Org syntax.
 CONTENTS is the contents of the element."
   (with-temp-buffer
+    (org-mode)
     (insert contents)
     (goto-char (point-min))
     (org-list-repair)
@@ -8031,6 +8032,9 @@ This function may modify the match data."
   (if (org-element-type epom t) epom
     (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))
       ;; Allow re-parsing when the command can benefit from it.
       (when (and cached-only
                  (memq this-command org-element--cache-non-modifying-commands))

testing/lisp/test-ox.el

@@ -1498,6 +1498,7 @@ Footnotes[fn:2], foot[fn:test] and [fn:inline:inline footnote]
       (let ((buffer (find-file-noselect includer t)))
 	(unwind-protect
 	    (with-current-buffer buffer
+              (org-mode)
 	      (org-export-expand-include-keyword)
 	      (org-trim (buffer-string)))
 	  (when (buffer-live-p buffer)
@@ -1520,6 +1521,7 @@ Footnotes[fn:2], foot[fn:test] and [fn:inline:inline footnote]
       (let ((buffer (find-file-noselect includer t)))
 	(unwind-protect
 	    (with-current-buffer buffer
+              (org-mode)
 	      (org-export-expand-include-keyword)
 	      (org-trim (buffer-string)))
 	  (when (buffer-live-p buffer)
@@ -1542,6 +1544,7 @@ Footnotes[fn:2], foot[fn:test] and [fn:inline:inline footnote]
       (let ((buffer (find-file-noselect includer t)))
 	(unwind-protect
 	    (with-current-buffer buffer
+              (org-mode)
 	      (org-export-expand-include-keyword)
 	      (org-trim (buffer-string)))
 	  (when (buffer-live-p buffer)
@@ -1564,6 +1567,7 @@ Footnotes[fn:2], foot[fn:test] and [fn:inline:inline footnote]
       (let ((buffer (find-file-noselect includer t)))
 	(unwind-protect
 	    (with-current-buffer buffer
+              (org-mode)
 	      (org-export-expand-include-keyword)
 	      (org-trim (buffer-string)))
 	  (when (buffer-live-p buffer)
@@ -1586,6 +1590,7 @@ Footnotes[fn:2], foot[fn:test] and [fn:inline:inline footnote]
       (let ((buffer (find-file-noselect includer t)))
 	(unwind-protect
 	    (with-current-buffer buffer
+              (org-mode)
 	      (org-export-expand-include-keyword)
 	      (org-trim (buffer-string)))
 	  (when (buffer-live-p buffer)
@@ -1608,6 +1613,7 @@ Footnotes[fn:2], foot[fn:test] and [fn:inline:inline footnote]
       (let ((buffer (find-file-noselect includer t)))
 	(unwind-protect
 	    (with-current-buffer buffer
+              (org-mode)
 	      (org-export-expand-include-keyword)
 	      (org-trim (buffer-string)))
 	  (when (buffer-live-p buffer)
@@ -1628,6 +1634,7 @@ Footnotes[fn:2], foot[fn:test] and [fn:inline:inline footnote]
      (let ((buffer (find-file-noselect includer t)))
        (unwind-protect
 	   (with-current-buffer buffer
+             (org-mode)
 	     (org-export-expand-include-keyword)
 	     (org-trim (buffer-string)))
 	 (when (buffer-live-p buffer)