[9.x] Added 'throwIf' method to PendingRequest

[denniseilander]

In addition to this PR which allows adding the throw method to the PendingRequest, this PR adds the throwIf method to throw the exception if the condition evaluates to true.

// Throws RequestException
return Http::baseUrl('https://foo.bar')
       ->throwIf(true)
       ->get('not-found');

// Doesn't throw
return Http::baseUrl('https://foo.bar')
       ->throwIf(false)
       ->get('not-found');