Add error message (#6500)

### Motivation and context
When users try to backup a task with cloud storage data they get such
error:

![Screenshot from 2023-07-18
09-21-18](https://github.com/opencv/cvat/assets/43179655/a6e4ed09-8fb5-4ebb-a5e3-72b95d9aa195)

And after that, they have no idea what's wrong with this task and why
they cannot backup it.

### How has this been tested?
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### Checklist
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- [ ] I submit my changes into the `develop` branch
- [ ] I have added a description of my changes into the
[CHANGELOG](https://github.com/opencv/cvat/blob/develop/CHANGELOG.md)
file
- [ ] I have updated the documentation accordingly
- [ ] I have added tests to cover my changes
- [ ] I have linked related issues (see [GitHub docs](

https://help.github.com/en/github/managing-your-work-on-github/linking-a-pull-request-to-an-issue#linking-a-pull-request-to-an-issue-using-a-keyword))
- [ ] I have increased versions of npm packages if it is necessary

([cvat-canvas](https://github.com/opencv/cvat/tree/develop/cvat-canvas#versioning),

[cvat-core](https://github.com/opencv/cvat/tree/develop/cvat-core#versioning),

[cvat-data](https://github.com/opencv/cvat/tree/develop/cvat-data#versioning)
and

[cvat-ui](https://github.com/opencv/cvat/tree/develop/cvat-ui#versioning))

### License

- [ ] I submit _my code changes_ under the same [MIT License](
https://github.com/opencv/cvat/blob/develop/LICENSE) that covers the
project.
  Feel free to contact the maintainers if that's a concern.

cvat/apps/engine/backup.py

@@ -364,7 +364,7 @@ def _write_data(self, zip_object, target_dir=None):
                 target_dir=target_data_dir,
             )
         else:
-            raise NotImplementedError()
+            raise NotImplementedError("We don't currently support backing up tasks with data from cloud storage")
 
     def _write_task(self, zip_object, target_dir=None):
         task_dir = self._db_task.get_dirname()