Input validation

Commit by commit review
Linked to https://www.agileplannerapp.com/boards/105200/cards/6340
Created a follow up card for the error message discussion: https://www.agileplannerapp.com/boards/105200/cards/8042 => @dsingleton let's check it out when you're back in, added to bottom of backlog as I judge it a non-blocker for go-live on SPL