Fix the impersonation token error when site_admin scope is missing

[ajaykn]

A recent PR: #46 added the support of impersonation.
For this, we need site_admin permissions needed.

When checking the site_admin scope, if missing, the error is wrapped with err
return "", errors.Wrap(err, "the current token does..)
But this doesn't work if err is nil.

we have to throw a new error in this case.

Before fix:

getting an impersonation token for `actions-admin` ...
these are the scopes we have for the current token `admin:org, repo` ...
syncing `actions/setup-node`
error creating github repository `actions/setup-node`: error retrieving authenticated user: GET https://h2floh-080c6ccdb7ad8f2c4.ghe-test.com/api/v3/user: 401 Bad credentials []

After fix:

getting an impersonation token for `actions-admin` ...
these are the scopes we have for the current token `admin:org, repo` ...
error obtaining the impersonation token: the current token doesn't have the `site_admin` scope, the impersonation function requires the `site_admin` permission to be able to impersonate.

Also in Readme.md, moved optional argument (--actions-admin-user) after required fields section.

[nicholasbergesen]

lgtm