org-timer.el: Allow org-timer-set-timer from non-Org buffers

* lisp/org-timer.el (org-timer--get-timer-title): If the current
buffer is not an Org buffer, use the buffer name as the timer title.

Currently all of the `org-timer-' operations work from any buffer
except `org-timer-set-timer' which must be run from an Org buffer.
This is because `org-timer-set-timer' sets a timer name based on an
Org heading or filename.  By setting the timer title to the current
buffer name we can use `org-timer-set-timer' from any buffer and
preserve the timer naming convention of using the buffer name if there
isn't an Org header.

TINYCHANGE

lisp/org-timer.el

@@ -466,7 +466,8 @@ time is up."
 		 (run-hooks 'org-timer-done-hook)))))
 
 (defun org-timer--get-timer-title ()
-  "Construct timer title from heading or file name of Org buffer."
+  "Construct timer title.
+Try to use an Org header, otherwise use the buffer name."
   (cond
    ((derived-mode-p 'org-agenda-mode)
     (let* ((marker (or (get-text-property (point) 'org-marker)
@@ -482,7 +483,7 @@ time is up."
    ((derived-mode-p 'org-mode)
     (or (ignore-errors (org-get-heading))
 	(buffer-name (buffer-base-buffer))))
-   (t (error "Not in an Org buffer"))))
+   (t (buffer-name (buffer-base-buffer)))))
 
 (provide 'org-timer)