better diagnostic for write failures during registry download #2626
Description
As I commented on discourse, we got a mysterious ERROR: could not download message during registry installation from a fresh Julia download (empty .julia), even though the download progress bar apparently completed and a manual download call succeeded for the same URL.
Switching to JULIA_PKG_SERVER="" yielded a more informative error from git clone: the problem was that the /tmp directory had a quota that we were exceeding (perhaps not from the download itself but from a subsequent unpacking step?).
It would be good if the pkgserver-based registry installation provided more informative error messages for write failures like this. ("could not download" sounds like misleadingly like a network problem.)