Handle exit codes in scripts

Use tailor's exit codes to distinguish configuration problems from build failures in CI.

Code	Meaning
0	Success.
1	Build or operational failure, such as an engine, IO, resolution, or signing error.
2	Usage or configuration error, such as bad arguments, an invalid selector, an unknown image, or a dependency cycle.
130	Interrupted run, such as Ctrl+C or SIGTERM cancellation (128 + SIGINT).

Branch on $? after a failed command:

if tailor build gizmo; then
  echo "build succeeded"
else
  code=$?
  case "$code" in
    2)
      echo "fix tailor.yaml or the build arguments"
      ;;
    1)
      echo "build failed; inspect the build log"
      ;;
    130)
      echo "build interrupted"
      ;;
  esac
  exit "$code"
fi

Related: see the CLI reference.