throw more specific exceptions; caller handles as desired

[ndushay]

This is in contradiction to merged PR #41

Possibly diff flavors of exceptions

• invalid year (e.g. 2050)
• unparseable year
• last year earlier than first year

etc.

[mjgiarlo]

@aaron-collier @ndushay Giving up on this one. I suspect it'll be easier to disentangle validation and parsing after another round or two of refactoring, so leaving the issue open. I moved the issue to the backlog.