Improve error message for missing plugin .mjs file

[agoose77]

When running through including a new transform to mystmd with @JimMadge, we observed that the error message for a non-existent plugin file isn't clearly referring to a plugin!

[JimMadge]

Example output,

❯ npx myst
...
ENOENT: no such file or directory, stat './no-alt-text.mjs'

with a configuration like,

version: 1
project:
  id: ...
  github: https://github.com/...
  plugins:
    - ./no-alt-text.mjs
site:
  template: book-theme