Problem/Motivation
Currently all messages are called using drupal_set_message(). This makes it difficult to unit test code that calls it.
Proposed resolution
Create a Message trait that wraps drupal_set_message and drupal_get_message in OO code, so it can be used by ControllerBase and FormBase.
Remaining tasks
Once #2278383: Convert drupal_set_message() to a service is in, this can be replaced with a call to the Messeger service.