Add validations and error messages to projects

https://trello.com/c/mzvoxpQB/44-add-error-messages-to-project-form

![selection_027](https://cloud.githubusercontent.com/assets/8248100/5108198/2845279e-6fd6-11e4-9b2e-47d719addd58.png)

Currently you do not permit `:completed`. Is this purposeful?