where in your pipe you got a non-null exit code

First thing you want is set -e and set -o pipefail. That should report the errors in human-parseable form.

Second, to capture exit codes from each command/program, you have to run each of them in sequence yourself, connected by pipes that you create via mkfifo — the same way as you would do it in any other programming environment. Bash's | pipes are just a convenient shorthand for this,so if you want full control, you have to ditch the convenience.