Revert TodoList changes if Transaction fails

This will prevent from leaving TodoList and todo.txt file in
inconsistent state whenever first Command.execute() inside Transaction
succeeds and any subsequent fails for whatever reason.

topydo/ui/columns/Main.py

@@ -324,6 +324,8 @@ class UIApplication(CLIApplicationBase):
         try:
             if transaction.execute():
                 self._post_execute()
+            else:
+                self._rollback()
         except TypeError:
             # TODO: show error message
             pass
@@ -341,6 +343,12 @@ class UIApplication(CLIApplicationBase):
         if dirty or self.marked_todos:
             self._reset_state()
 
+    def _rollback(self):
+        try:
+            self.backup.apply(self.todolist, p_archive=None)
+        except AttributeError:
+            pass
+
     def _repeat_last_cmd(self, p_todo_id=None):
         try:
             cmd, verbosity = self._last_cmd