Fix app entrypoint bug in RN gestorybook

[shilman]

Issue: #1277

What I did

• replace APP_NAME in the correct file
• add a warning if APP_NAME cannot be found

How to test

Run getstorybook in a fresh RN app and check the results

[storybookbot]

	Fails
🚫	PR is not labeled with one of: ["cleanup","breaking","feature","bug","documentation","maintenance","greenkeeper","other"]

Generated by 🚫 dangerJS

[codecov]

Codecov Report

Merging #1280 into master will increase coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1280      +/-   ##
==========================================
+ Coverage   13.71%   13.73%   +0.02%     
==========================================
  Files         207      207              
  Lines        4646     4638       -8     
  Branches      526      638     +112     
==========================================
  Hits          637      637              
+ Misses       3542     3443      -99     
- Partials      467      558      +91

Impacted Files	Coverage Δ
addons/storyshots/src/test-bodies.js	0% <0%> (ø)	⬆️
...react-native/src/manager/components/PreviewHelp.js	0% <0%> (ø)	⬆️
lib/ui/src/modules/shortcuts/actions/shortcuts.js	6.25% <0%> (ø)	⬆️
addons/storyshots/src/require_context.js	0% <0%> (ø)	⬆️
...src/server/config/WatchMissingNodeModulesPlugin.js	0% <0%> (ø)	⬆️
addons/info/src/components/Node.js	0% <0%> (ø)	⬆️
addons/info/src/components/PropVal.js	0% <0%> (ø)	⬆️
addons/knobs/src/components/PropField.js	10.86% <0%> (ø)	⬆️
addons/knobs/src/KnobManager.js	38.57% <0%> (ø)	⬆️
addons/info/src/components/markdown/code.js	0% <0%> (ø)	⬆️
... and 15 more

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[tmeasday]

Perfect! Great job