"Download Failed" message is unhelpful when the underlying error is HTTP 404
I recently encountered a situation where I was trying to create an express-install Ubuntu VM in Boxes and it failed, toasting the message "Download Failed". This message does not tell me a whole lot about what went wrong. In particular, it does not tell me whether this error is transient or likely to be permanent.
Running Boxes from the terminal revealed that the underlying error was a 404 Not Found response from the server; i.e. likely permanent. This should have been reported to me in some helpful way so I could tell that it wasn't just because e.g. my network connection was flaky.
(FWIW I am planning to report the specific 404 problem to my distribution first, instead of here, since I worry it's just because Boxes is outdated.)