`org-forward/backward-paragraph' do not error at buffer boundaries

* lisp/org.el (org-forward-paragraph):
(org-backward-paragraph): Do not error at buffer boundaries.

Reported-by: Omar Antolín Camarena <omar.antolin@gmail.com>
<http://lists.gnu.org/archive/html/emacs-orgmode/2017-07/msg00478.html>

lisp/org.el

@@ -24432,7 +24432,7 @@ item, etc.  It also provides some special moves for convenience:
   - On a table or a property drawer, jump after it.
   - On a verse or source block, stop after blank lines."
   (interactive)
-  (when (eobp) (user-error "Cannot move further down"))
+  (unless (eobp)
     (let* ((deactivate-mark nil)
 	   (element (org-element-at-point))
 	   (type (org-element-type element))
@@ -24499,7 +24499,7 @@ item, etc.  It also provides some special moves for convenience:
 	    ((> (line-end-position) contents-begin)
 	     (end-of-line)
 	     (org-forward-paragraph))
-          (t (goto-char contents-begin)))))
+	    (t (goto-char contents-begin))))))
 
 (defun org-backward-paragraph ()
   "Move backward to start of previous paragraph or equivalent.
@@ -24514,7 +24514,7 @@ convenience:
   - On a table or a property drawer, move to its beginning.
   - On a verse or source block, stop before blank lines."
   (interactive)
-  (when (bobp) (user-error "Cannot move further up"))
+  (unless (bobp)
     (let* ((deactivate-mark nil)
 	   (element (org-element-at-point))
 	   (type (org-element-type element))
@@ -24562,7 +24562,7 @@ convenience:
 	(org-backward-paragraph))
        (t (goto-char (or post-affiliated begin))))
       ;; Ensure we never leave point invisible.
-    (when (org-invisible-p (point)) (beginning-of-visual-line))))
+      (when (org-invisible-p (point)) (beginning-of-visual-line)))))
 
 (defun org-forward-element ()
   "Move forward by one element.

testing/lisp/test-org.el

@@ -3259,11 +3259,12 @@ SCHEDULED: <2017-05-06 Sat>
 
 (ert-deftest test-org/forward-paragraph ()
   "Test `org-forward-paragraph' specifications."
-  ;; At end of buffer, return an error.
-  (should-error
+  ;; At end of buffer, do not return an error.
+  (should
    (org-test-with-temp-text "Paragraph"
      (goto-char (point-max))
-     (org-forward-paragraph)))
+     (org-forward-paragraph)
+     t))
   ;; Standard test.
   (should
    (org-test-with-temp-text "P1\n\nP2\n\nP3"
@@ -3328,10 +3329,11 @@ SCHEDULED: <2017-05-06 Sat>
 
 (ert-deftest test-org/backward-paragraph ()
   "Test `org-backward-paragraph' specifications."
-  ;; Error at beginning of buffer.
-  (should-error
+  ;; Do not error at beginning of buffer.
+  (should
    (org-test-with-temp-text "Paragraph"
-     (org-backward-paragraph)))
+     (org-backward-paragraph)
+     t))
   ;; Regular test.
   (should
    (org-test-with-temp-text "P1\n\nP2\n\nP3"