Rename GenericProcessingReport to AbstractProcessingReport

Signed-off-by: Francis Galiegue <fgaliegue@gmail.com>
Create a ListProcessingReport class

We will very often want to collect messages in a List, so create an intermediary
class.

Make SyntaxReport use that class.

Signed-off-by: Francis Galiegue <fgaliegue@gmail.com>