Curl missing not resulting in error message when calling :TypstPreviewUpdate

[asmund20]

Running :TypstPreviewUpdate on a system without curl installed does not result in an error message, at least that was the case with my Ubuntu 24.04 TLS and neovim 0.10.1.

I was going to try making a PR for this, but as far as I can tell, the

vim.loop.spawn('curl' ...

on line 130 in fetch should result in an exit code that is non-zero and thus an error-message logged. Due to me not really understaning what is going on, I am making this issue hoping that someone fixes it, or that it can help others with the same problems.

It could be enough to clarify that curl is a dependency in the readme/docs, but would be nice to have error-handling for that.