Warn or throw error for tab bar icon

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

Start by locating the JavaScript handling for tab bar icons and the TabBarIOS integration in this package. Reproduce the unsupported usage, then add a clear warning or error when it is detected; done means users are told that this combination will not work.

Written by the indexing model from the issue text.