download-plugins: fix logging of failures

[perrinjerome]

What it does

Because Array.forEach(f) calls f with (currentValue, index, array), the output
was not only the messages, but also the index and full array.

This changes to log only the message.

How to test

Change theiaPlugins in package.json to something like this:

  "theiaPlugins": {
    "error-404": "https://open-vsx.org/404.vsix",
    "another-error": "https://open-vsx.org/404.vsix"
  }

run yarn download:plugins. Without this change, in the output there is the index and the array of failures, it looks like this:

After this change, the output is:

Review checklist

• as an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• as a reviewer, I agree to behave in accordance with the review guidelines

[paul-marechal]

Good catch, LGTM.

[perrinjerome]

I rebased to solve conflicts, now that #8788 is merged