AccountService: Implements default account failsafe

Implements one of the fixes outlined in issue #2637.

/cc @sendhil + @astralbodies + @koke 

Then IMO it should be removed. I'm assuming `println` works as `NSLog` and it has no place in production code. https://github.com/wordpress-mobile/WordPress-iOS/wiki/WordPress-for-iOS-Objective-C-Style-Guide#before-you-commit