[8.x] add common 4xx error status code boolean methods to response class

[hendeavors]

This adds methods to check 401 and 403 status codes, respectively. I think these are pretty common status codes to work with when consuming an api.

Before:

if ($response->status() === 401) {
    // ...
}

if ($response->status() === 403) {
    // ...
}

After:

if ($response->unauthorized()) {
    // ...
}

if ($response->forbidden()) {
    // ...
}