fix: check if file exits before downloading

[hamza221]

Summary

Make sure that files exist before downloading

reproduction

Scenario 1:

1. Share a file with user1
2. login as user 1
3. delete share
4. don't refresh page as user1
5. download file
6. opening/downloading the "missing" shared file causes "Page not found" errors

Scenario 2:

1. create a federated share
2. receiver can open the share
3. federated share is removed by owner
4. receiver does not refresh the folder and so does not recognize the removal
5. opening/downloading the "missing" shared file causes "Page not found" errors

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)

[susnux]

What is fixed by this? For me it seems like a lot of unnecessary requests to the server.
Imagine to download a full server, you would do like X expensive PROPFIND just to the do a GET request.

Instead the GET should just fail with 404 which is an expected behavior.
IMHO a proper fix would be to have to kind of notification of changes causing the webui to reload, just like mobile clients do.

Meaning I do not see a benefit here expect from spamming the server with unneeded requests.

[susnux]

(federated share) is removed by owner
receiver does not refresh the folder and so does not recognize the removal

This can also happen between your PROPFIND and the real download, this does not ensure that it exists. Its not locked or similar.

[hamza221]

As mentioned above this would cause performance issues and still doesn't solve all edge cases. an adequate solution would be implementing #2742