Last command's exit status, for your prompt. Put ${last_exit} into your prompt somewhere. It uses zsh's prompt escapes.

exit-status.plugin.zsh

precmd_functions=("_exit_status" ${precmd_functions[@]})
function _exit_status() {
  local last_exit_status=$?

  export last_exit=""
  if [ -z "${last_exit_status}" ]; then
    last_exit=""
  elif (( last_exit_status != 0 )); then
    local description

    # Let's be comprehensive
    case $last_exit_status in
      # Signals are 128 + signal value, we translate back to the signal name
      $((128 + 1))  ) description="SIGHUP";;
      $((128 + 2))  ) description="SIGINT";;
      $((128 + 3))  ) description="SIGQUIT";;
      $((128 + 4))  ) description="SIGILL";;
      $((128 + 5))  ) description="SIGTRAP";;
      $((128 + 6))  ) description="SIGABRT";;
      $((128 + 7))  ) description="SIGEMT";;
      $((128 + 8))  ) description="SIGFPE";;
      $((128 + 9))  ) description="SIGKILL";;
      $((128 + 10)) ) description="SIGBUS";;
      $((128 + 11)) ) description="SIGSEGV";;
      $((128 + 12)) ) description="SIGSYS";;
      $((128 + 13)) ) description="SIGPIPE";;
      $((128 + 14)) ) description="SIGALRM";;
      $((128 + 15)) ) description="SIGTERM";;
      $((128 + 16)) ) description="SIGURG";;
      $((128 + 17)) ) description="SIGSTOP";;
      $((128 + 18)) ) description="SIGTSTP";;
      $((128 + 19)) ) description="SIGCONT";;
      $((128 + 20)) ) description="SIGCHLD";;
      $((128 + 21)) ) description="SIGTTIN";;
      $((128 + 22)) ) description="SIGTTOU";;
      $((128 + 23)) ) description="SIGIO";;
      $((128 + 24)) ) description="SIGXCPU";;
      $((128 + 25)) ) description="SIGXFSZ";;
      $((128 + 26)) ) description="SIGVTALRM";;
      $((128 + 27)) ) description="SIGPROF";;
      $((128 + 28)) ) description="SIGWINCH";;
      $((128 + 29)) ) description="SIGINFO";;
      $((128 + 30)) ) description="SIGUSR1";;
      $((128 + 31)) ) description="SIGUSR2";;
      # All other cases. While ZSH may have specific errors for lack of permissions
      # the script itself may define these, to be conservative, just output the
      # number.
      *)              description="${last_exit_status}";;
    esac
    last_exit="%F{red}${description}%f "
  fi;
}