Improve check_for_archived_meeting

[vkrasnovyd]

Incorrect error message can be raised on attempt to perform action on a meeting that does not exist. Below is one of the examples.

How to reproduce:
Open /motion/test_create_forwarded.py and modify request in test_forward_with_attachment_true_and_forward_with_attachments_disabled: replace meeting_id in `self.request1 with an id of a meeting that does not exist.

Current error message:

Meeting /2435 cannot be changed, because it is archived.

Desired behaviour:
Message to contain the information that target meeting does not exist.