Hint about a possible update failure reason

This feels ugly because it doesn't fix the underlying bug, but
it's incredibly hard to trace down the root cause in this
situation and it causes updates to fail which is pretty
critical. So this is probably better than nothing.

Ref QubesOS/qubes-issues#5052

package-managers/qubes-download-dom0-updates.sh

@@ -208,6 +208,7 @@ if ls "$DOM0_UPDATES_DIR"/packages/*.rpm > /dev/null 2>&1; then
     $cmd "$DOM0_UPDATES_DIR"/packages/*.rpm || { qrexec_exit_code=$? ; true; };
     if [ ! "$qrexec_exit_code" = "0" ]; then
         echo "'$cmd $DOM0_UPDATES_DIR/packages/*.rpm' failed with exit code ${qrexec_exit_code}!" >&2
+        echo 'Sometimes this is caused by dom0 not having enough free space.' >&2
         exit "$qrexec_exit_code"
     fi
 else