File uploads: handle upload errors

[winged]

When a file upload fails (for example, file size too large), a document still is created, but for obvious reasons cannot be downloaded.

First, we need to make the error more prominent in the frontend, so the user will notice something went wrong.

Second, and more importantly, we need to think about handling the error:

A) in a new upload, we may delete the document again. Which component triggers this is yet to-be determined. I could think about the backend checking file presence after a while, or the frontend explicitly checking the upload's success and taking appropriate measures.

B) When uploading a new version, only the file in the document may be deleted

[Yelinz]

Addressed by #507

Largest problem with the empty document is fixed, but smaller ones may still persist