Warn or throw error for tab bar icon

[brentvatne]

expo/vector-icons doesn't play nicely with TabBarIOS because 1) we don't recommend using it, JS solution is cross-platform and more customizable 2) we don't want to add the native module necessary to make it play nicely to Exponent.

So we should make it very clear whenever someone tries this that it will not work

[anlumo]

I'd be happy to hear your suggestions on what exactly to use instead of TabBarIOS. I'm unable to find any component in React Native that reimplements its behavior in JavaScript.

This restriction means that I simply can't use expo/vector-icons.

[brentvatne]

react-navigation