Fix some file renaming woes (probably)

[calumapplepie]

Hullo, new contributer, wanted to avoid others losing data the way I just did!

I added a few new checks that, to my knowlege, will prevent users from renaming files on top of files that exist. While this can be useful for reseeding, it causes instant data loss if qbittorrent has downloaded any part of the existing file.

I also do some cleanup on messages and exists() function.

Reject the PR if you'd prefer to move the functionality to a different place, but I had the itch to hack something up to fix this, and I couldn't resist.

Probably closes a few issues, but I wasn't sure which, and I should get back to final projects.

[luzpaz]

Thanks! Can you look at https://github.com/qbittorrent/qBittorrent/issues?q=is%3Aopen+is%3Aissue+label%3A%22Data+loss%22 and see which tickets this PR addresses ?

[github-actions[bot]]

This PR is stale because it has been 60 days with no activity. This PR will be automatically closed within 7 days if there is no further activity.

[calumapplepie]

Not stale, I just don’t have the bandwidth to hunt through issues in an unfamiliar repo.

[github-actions[bot]]

This PR is stale because it has been 60 days with no activity. This PR will be automatically closed within 7 days if there is no further activity.

[calumapplepie]

Not stale

[github-actions[bot]]

This PR is stale because it has been 60 days with no activity. This PR will be automatically closed within 7 days if there is no further activity.

[calumapplepie]

Still not stale <3