Die rather than warn if a Docker image is missing

The warning was done to handle older Nix releases that didn't have
Docker images (091f232), but this was
a bad idea because it causes us to silently skip uploading Docker
images if e.g. Hydra hasn't finished building them yet.

Issue NixOS#10648.

maintainers/upload-release.pl

@@ -189,10 +189,7 @@ sub downloadFile {
     eval {
         downloadFile("dockerImage.$system", "1", $fn);
     };
-    if ($@) {
-        warn "$@" if $@;
-        next;
-    }
+    die "$@" if $@;
     $haveDocker = 1;
 
     print STDERR "loading docker image for $dockerPlatform...\n";