🐛 Improve error message for missing javascript and python plugins

[JimMadge]

Closes #1813

It looks like the problem was here

mystmd/packages/myst-cli/src/plugins.ts

Line 87 in d1f8768

if (!fs.statSync(path).isFile || !path.endsWith('.mjs')) {

fs.statSync raises an error if there is no file/directory.
This PR adds an option so that fs.statSync returns undefined if there is no file, then handles that possibility when calling isFile().

I see that there is another instance of fs.statSync(...).isFile() in plugins.ts that might cause similar problems.

[changeset-bot]

🦋 Changeset detected

Latest commit: dbe6279

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 2 packages

Name	Type
myst-cli	Patch
mystmd	Patch

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[rowanc1]

Thanks, this looks good. Will merge when tests go green.

Thanks @JimMadge.

[rowanc1]

Have expanded to cover the python ones as well. cc @agoose77.