Annotate cells that are expected to error as a form of (unit-)testing

[rodrigogiraoserrao]

Following up on #21, it would be great if there was some way of annotating cells that are expected to error. Ideally, the build system would take that to mean that a cell errors if and only if it is annotated as such; if this condition is not met, building the book would fail and we'd get a report showing what went wrong, where.