Don't try to convert to linenumber when a textual UID is passed

When a UID of 1aa is passed, the code path would try to convert it to a
linenumber (integer) when linenumbers are configured. That fails and
should be caught accordingly.

test/test_todo_list.py

@@ -219,6 +219,13 @@ class TodoListTester(TopydoTest):
         config("test/data/todolist-uid.conf")
         self.assertRaises(InvalidTodoException, self.todolist.todo, 1)
 
+    def test_uid4(self):
+        """
+        Handle UIDs properly when line numbers are configured.
+        """
+        config(p_overrides={('topydo', 'identifiers'): 'linenumber'})
+        self.assertRaises(InvalidTodoException, self.todolist.todo, '11a')
+
     def test_new_uid(self):
         """ Make sure that item has new text ID after append. """
         config("test/data/todolist-uid.conf")

topydo/lib/TodoListBase.py

@@ -98,7 +98,7 @@ class TodoListBase(object):
                 except TypeError:
                     try:
                         result = self._todos[int(p_identifier) - 1]
-                    except IndexError:
+                    except (ValueError, IndexError):
                         raise InvalidTodoException
 
             return result