AuthComponent: Allow suppressing authError message

When unauthenticated users accesses protected areas, they are greeted
with the default `You are not allowed to access that location` which is
not desired in some cases.

This patch allows applications to suppress this message by setting
`AuthComponent::authError` to `false` bypassing the call to
`SessionComponent::setFlash()` altogether.

Refs: https://github.com/croogo/croogo/pull/175#discussion_r4714240

Note: Please fast-forward and don't use the merge button :)