[TASK] More descriptive error message on file upload failure

Provide the user with more accurate information about what might have
gone wrong.

Resolves: #46160
Releases: 6.0,6.1
Change-Id: I86123b904f36ff7f6db77f2dd13f97c0a01957c3
Reviewed-on: https://review.typo3.org/18840
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn