Errors from recursive-readdir are ignored

[GMNGeoffrey]

My workspace has some weird files in the repository due to bazel builds. I didn't properly exclude them (because it looks like exclusions are expected to match on the absolute path, not relative to the workspace root, as I expected, #17). The error I get is the rather misleading Warning, no matches were found. I was able to debug the issue by fiddling with the local install of the extension. It looks like there's a readErr there, but it's unused. Maybe it should be surfaced to the user.

I'd think that that a simple

if (readErr) {
  showError(readErr);
}

would do it, but it looks like in the error case that callback gets called multiple times and since the error messages are different, vscode doesn't de-dupe them and crashes. What about converting to the promise syntax and using that?

      recursive(folderFullPath, searchExclusion,).then((files) => {
          // ...
        }, (err) => {
         showError(err);
        }
      );

Then everything only gets called once. Happy to send a PR