org-habit.el: better error handling required

Attached is the patch which catch this error and throws meaningful
message.

* lisp/org-habit.el (org-habit-parse-todo): Find sr-days only if
scheduled-repeat is non nil.  Use 4th element of the list returned
by (org-heading-components) as habit-entry.  Modify the error
message to be more meaningful.

TINYCHANGE

paulusm <paulusm@bigpond.com> writes:

> Hi org-mode people,
>
> Whilst playing with the "shaving" example from
> http://orgmode.org/manual/Tracking-your-habits.html I accidentally put a
> bad character in the SCHEDULED timestamp.
>
> Instead of: "SCHEDULED: <2010-08-26 Thu .+2d/4d>"
> I had:      "SCHEDULED: <2010-08-26 Thu .+2nd/4d>"
>
> When trying to view my agenda, I was presented with a blank agenda and Emacs
> very quietly reported:
>     "org-habit-duration-to-days: Wrong type argument: stringp, nil"
> which is not really helpful.
>
> Removing the bad character fixes the issue, and I can duplicate the error
> condition as described above.
>
> Perhaps some better error trapping could be done?
>
>

lisp/org-habit.el

@@ -149,15 +149,17 @@ This list represents a \"habit\" for the rest of this module."
     (assert (org-is-habit-p (point)))
     (let* ((scheduled (org-get-scheduled-time (point)))
 	   (scheduled-repeat (org-get-repeat org-scheduled-string))
-	   (sr-days (org-habit-duration-to-days scheduled-repeat))
 	   (end (org-entry-end-position))
-	   (habit-entry (org-no-properties (nth 5 (org-heading-components))))
-	   closed-dates deadline dr-days)
+	   (habit-entry (org-no-properties (nth 4 (org-heading-components))))
+	   closed-dates deadline dr-days sr-days)
       (if scheduled
 	  (setq scheduled (time-to-days scheduled))
 	(error "Habit %s has no scheduled date" habit-entry))
       (unless scheduled-repeat
-	(error "Habit %s has no scheduled repeat period" habit-entry))
+	(error
+	 "Habit '%s' has no scheduled repeat period or has an incorrect one"
+	 habit-entry))
+      (setq sr-days (org-habit-duration-to-days scheduled-repeat))
       (unless (> sr-days 0)
 	(error "Habit %s scheduled repeat period is less than 1d" habit-entry))
       (when (string-match "/\\([0-9]+[dwmy]\\)" scheduled-repeat)