notmuch_database_create: Document idea to (optionally) return a status

The current problem is that when this function fails the caller
doesn't get any information about what the particular failure
was, (something in the filesystem? or in Xapian?). We should fix
that.

notmuch.h

@@ -144,6 +144,9 @@ notmuch_database_default_path (void);
 notmuch_database_t *
 notmuch_database_create (const char *path);
 
+/* XXX: I think I'd like this to take an extra argument of
+ * notmuch_status_t* for returning a status value on failure. */
+
 /* Open a an existing notmuch database located at 'path'.
  *
  * The database should have been created at some time in the past,