[v6] Upload dialog doesn't show upload errors #8645

Description

When an upload fails, the upload dialog shows no error and nothing happens. E.g. uploading a file whose name already exists: the server answers 400 with "A file with the name "test.mp4" already exists", but as a Panel user I don't see anything.

Expected behavior
The error is shown below the file, like in Kirby 5.

Screenshots

Kirby 5.5.3:

Kirby 6.0.0-beta.1

Yes, I pressed "Upload" :)

To reproduce

1. Plainkit with a filelist field and a file test.mp4 on the page
2. Upload another file named test.mp4
3. Click "Upload": no error is shown

Your setup

Kirby Version
6.0.0-beta.1

Console output
The upload request fails with 400 (Bad Request), no other errors.

Your system (please complete the following information)

• OS: macOS
• Browser: Helium
• Version: 0.19.2.1

Additional context
Works in 5.5.3.