Better error management and removal support in baking/processing. * Baker and processor pipeline now store errors in their records. * They also support deleting output files that are no longer valid. * The basic transitional record class implements more boilerplate code. * The processor pipeline is run from the `bake` command directly. * New unit tests. * Unit test mocking now mocks `os.remove` too.

*.pyc
venv/
build/lib
build/messages/_cache
dist/
piecrust.egg-info/
piecrust/__version__.py