pkg repo signing_command: Detect if the command fails and fail early
pkg repo signing_command: Detect if the command fails and fail early

Also cleanup exit path in cmd_sign() while here.

Open343/pkg,skoef/pkg,khorben/pkg,en90/pkg,khorben/pkg,junovitch/pkg,skoef/pkg,en90/pkg,Open343/pkg,junovitch/pkg,khorben/pkg