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