Add sanity check for cml-publish with filesystem paths

[0x2b3bfa0]

Resolves #308 and probably also closes #401 by throwing a meaningful error for paths that don't exist, aren't files or can't be read.

[DavidGOrtega]

Let me check this. But the issue is that in the command line there is no difference between a buffer and a path. I mean you can do:

• cml-publish path
• pipe a buffer

[0x2b3bfa0]

@DavidGOrtega, both of my proposed solutions consider the buffer/path dichotomy, taking care only of the latter.

[DavidGOrtega]

Nice! Can we add a small test with a non existent file? Also with a a buffer would be awesome.
If not maybe opening a ticket would be also very nice!

[0x2b3bfa0]

I finally chose to call stat before calling mime_type so the former provides the safeguard and the meaningful message. If you want to offer a custom but more broad error message, we can revert to the previous solution:

if (path) {
  try {
    await fs.promises.access(path, fs.constants.F_OK | fs.constants.R_OK);
  } catch (err) {
    throw new Error(`Path ${path} does not exist or is not a readable file.`);
  }
}

Otherwise, please review the latest changes and approve the pull request if applicable.

[DavidGOrtega]

lgtm 👍