org.el: Add more explicit errors. Fix some declarations and one autoload.

org.el: Add more explicit errors.  Fix some declarations and one autoload.

* org.el (org-refile-check-position): Throw an error when the
refile target is the current buffer and is not a file.
(org-agenda-file-to-front, org-remove-file): Throw an error
when the current buffer is not a file.
(org-check-agenda-file): Enhance the message.
(org-element-type): Autoload.
(org-element-context, org-element-paragraph-parser): Don't
declare as these two functions are not used in org.el.