Martin Konicek a6adc501e8 Add a hint to a very common AppRegistry error
Summary:
**Motivation (why should we merge this?)**

I see the following error very often when working on multiple apps and switching between them. In fact, switching between apps is the only reason why I ever saw the error. However, the error message is very unhelpful:

![screenshot 2017-02-22 16 15 29](https://cloud.githubusercontent.com/assets/346214/23221214/5f6bc7ba-f91c-11e6-9482-9183c27c5e9d.png)

**Test plan (required)**

![screenshot 2017-02-22 19 44 54](https://cloud.githubusercontent.com/assets/346214/23229247/830f7142-f937-11e6-9657-bad83563f3b0.png)
Closes https://github.com/facebook/react-native/pull/12517

Differential Revision: D4600082

Pulled By: mkonicek

fbshipit-source-id: 011c71dbac9e294348fd06c8e6d651228fac3288
2017-02-22 14:16:06 -08:00
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