make manifest download script handle errors correctly with dash

[jgressma]

/bin/sh points to dash on Ubuntu 18.04 LTS. The construct

{ VAR=1; } &>/dev/null

doesn't set VAR for dash but it does for bash. Previously the script would (benignly?) succeed, regardless of python's exit code. Here is a typical output prior to this patch:

• set +e
• python args ...
• [ -ne 0 ] manifest-setup.sh: 67: [: -ne: unexpected operator
• rm ./input-jrOOHByJvm.blf
• RETURNCODE=0
• set +x
• exit