stop_for_problems

[hadley]

By analogy to httr::stop_for_status() it would be useful to have a function that threw an error if there were any parsing problems. You'd use this in automated scripts if you wanted to be really sure that the file format hadn't changed

[hadley]

@earino - this allows you to be super strict about parsing. I think, combined with the new spec printing behaviour, this should make it easy to rapidly prototype the correct parsing specification and, later, fail-fast if the underlying data changes.