Unknown plot marks fail silently

[muzimuzhi]

Brief outline of the bug

An unknown plot mark should give some prompt instead of failing silently and drawing nothing.

Edit: Sorry for using the bug issue template. It can be considered as a feature request as well.

Minimal working example (MWE)

In example below, unknown marks o and alwaysunknown both fail silently.

\documentclass{article}
\usepackage{tikz}

\begin{document}
\tikz {
  \draw plot[mark=x] (0,0);
  \draw plot[mark=o] (1,0); % need libraray `plotmarks`
  \draw plot[mark=alwaysunknown] (2,0);
}
\end{document}

I've prepared an implementation on https://github.com/muzimuzhi/pgf/tree/unknown-mark.