ui/component should provide helpful message when component is not found

Currently, when referring to a component by key, you get the following:

template.cljs?rel=1461275696209:272 Uncaught Error: Assert failed: Invalid Hiccup form: [nil] (in reagent8 > reagent9 >

Would be much nicer if it behaved the same way as subscriptions, reporting that the component key does not exist in the registry.