Implementing validation for internal and parent controller scope

- translation.json updated with error message regarding spreadsheet needing to be published
- Service modified to remove catching the error promise in "getSheets" as this is handled by the directive
- Updated HTML template to use a div element with ng-if directive to conditionally show error message regarding a spreadsheet selected that has not been published
- Directive now optionally requires ngModel so that $setValidity can be called with the parent controller based on condition of spreadsheet url value
- Logic in place in directive to handle a spreadsheet selected that has not been published
Merge pull request #19 from stulees/chore-validation

Implementing validation for internal and parent controller scope