layers: Add default layer error message config

Allows layers to output error messages even if no vk_layer_settings.txt
config file is present. Sets defaults to LOG_MSG, error, stdout.
A layer settings file will override any default values. If no settings
file is present and an app creates a debug callback, the default
callbacks will be removed and unregistered.

Change-Id: I49f37189665816df58c258b9e9629f2bf76751c8