Refactor for testability and clearer error reporting

[Fred Emmott]

• need a clear concept of file-, class-, and method- level errors
• they need to be propagated upwards
• one file/class failing should be reported as a failure rather than breaking the whole lot
• don't report stack traces for reported errors
• replace passing a progress writer + $progress_writer->write($event_arg1, $event_arg2...) $writer->write($event) to enable a hierarchy of events with varying arguments