Make ProcessingReport an interface

I am too final happy. .error() being final, can't mock :(

What was ProcessingReport is now named GenericProcessingReport.

Signed-off-by: Francis Galiegue <fgaliegue@gmail.com>
Start to write tests for SyntaxChecker

Simple ones to start with.

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