Clean up some warnings that were hanging around

This refactors the front-end to use a tree for capturing errors, instead
of throwing exceptions.

This has the benefit that you will receive notifications about all of
the error messages you have, instead of only the first.

I'm a bit fuzzy on the details, since it's been a while since I wrote
these changes.

This is to prepare for replacing contents with a ReportWriter

The main refactoring here is removing the type field from the various
classes, but there are a few other smaller ones.

This also contains the grounds for a refactoring on variable use

RuleCase now handles its own generation.