Uploads without TUS always fail with "Unknown error" #4232

Initial Checklist

• I understand this is a bug report and questions should be posted in the Community Discussions
• I searched issues and couldn’t find anything (or linked relevant results below)

Bug Description

On a server without TUS support, failed uploads always say "Unknown error", also for a locked target, a full quota or a server message. getUploadItemMessage in UploadInfo.vue only reads the status from tus-js-client errors, while @uppy/xhr-upload puts it in error.request.

Reproduction Steps

1. Use a server without TUS (files.tus_support.max_chunk_size 0)
2. Upload onto a locked file, or with the quota exceeded
3. Open Show details in the upload panel

Expected Outcome

"This item is locked", "Quota exceeded" or the server's message, the same as with TUS.

Actual Outcome

"Unknown error" for 423, 507 and 500.