Don't fail silently when grammar is missing

[purcell]

These conditional clauses are found in some -ts modes, but only in the case where the parent mode is the non-ts version of the mode: in such cases, the clause serves to quietly "downgrade" the mode to the non-ts version when the grammar is missing, so that the -ts mode can be enabled unconditionally in auto-mode-alist.

In cases where the parent mode is just prog-mode, like noir-ts-mode, the -ts mode is non-functional without the grammar and should throw an error instead.

As a related note, it is unclear how best to handle the ###autoload-ed addition to auto-mode-alist, since we'd expect to be able to install the same package config in Emacsen with and without tree-sitter. It's probably safest to guard the auto-mode-alist expression in a (when (treesit-read-p 'lang) ...) form too.

(It looks like a couple of whitespace fixes found their way into the commit too, sorry.)

[hhamud]

Hey Purcell. Thanks for the contribution.

I have another noir mode here that is based on the Rust mode and I was thinking about a way to combine the ts-mode and non-ts mode packages together but I haven't had enough time to think it through and implement them.

Any suggestions would be welcome.

[purcell]

Yeah, then it sounds like you could probably follow the pattern of, say python-mode and python-ts-mode in the emacs standard library: make the ts mode package depend on the non-ts package, and use noir-mode as the parent of noir-ts-mode. If you did that, you'd keep the when clause that this PR removes.

[hhamud]

I think what I can do is merge the above changes when you've included this change

It's probably safest to guard the auto-mode-alist expression in a (when (treesit-read-p 'lang) ...) form too.

and then when I've updated the noir package down the line to update both packages.

[purcell]

That makes sense!