Don't let the user waste time when we know that the upload will fail

[tbsbdr]

Steps to reproduce

1. try to upload 50.000 files at once
2. Browser loads (user waits)
3. upload starts (user waits)
4. upload breaks at some point

Expected behaviour

• Don't let the user waste time when we know that the upload will fail.
• Warn the user when the amount of files to upload is likely to abort the upload.

Actual behaviour

Upload breaks at some point after the user has waited for some time for the upload.