[sendfile] Show error from send operation to the user

If the sending fails for whatever reason we say it has failed, but don't show any details, leaving the user wondering what's wrong

Not that dumping a technical error message to the user is great UX, but not showing it is worse IMO

This way the user can at least paste it into Google and maybe get something helpful or show a screenshot in a bug report
1 job for !58 with work/sendfileerror in 55 seconds (queued for 12 seconds)
latest merge request