`admin:org` is inclusive of `read:org`, so if we find the former listed
in response headers, we can conclude that the token has necessary scopes
instead of letting a warning notice be shown.
`fmt.Errorf` hides information and makes it hard to test for specific
conditions in returned error. Return a structured error instead.
Signed-off-by: Pavel Borzenkov <pavel.borzenkov@gmail.com>
