VSIX upload errors should indicate problematic filename(s)

[Alhadis]

Describe the bug

This week I attempted to upload a new release of a VS Code extension (file-icons.file-icons). Each attempt was met with the same nebulous error:

In this case, the "specified part" turned out to be a missing LICENSE.md (something that wasn't a blocker until recently). Had the error stated which file(s) I neglected to include in the .vsix, it would've saved days of frustration.

Expected behaviour

An error message that elucidates which "specified part" is being referred to.

Desktop

• OS: macOS 12.6.7 (Monterey), build 21G651
• Browser: Observed with Firefox 114.0.2 (default browser); Edge exhibits the same behaviour.

Additional context

I generate this .vsix manually in lieu of using vsce.

[agr]

The reference to LICENSE.md exists in the package manifest for about a year (since April, 5th, 2022), but copying the file itself was added to packaging target in Makefile only couple of days ago (I'd guess as an attempt to address the publishing failure). So, this is the web site complaining (in rather unhelpful way) about a file that is declared in manifest, but missing in the package. Assuming that "2 years ago" from the screenshot is actual previous publish time, that's not a new requirement that you ran into, but a result of a change of the manifest that happened after the package was published last (a year ago).

Regardless, the message needs to be clearer.