Summary:
Motivation:
`Invariant Violation: Module AppRegistry is not a registered callable module (calling runApplication).` is an error that happens rarely, and most SO answers end up recommending to use `react-native bundle` to work around. I stumbled upon this error the other day and it took me a while to figure out that the error was caused by the fact that my `entryFile` path was pointing to an existing but invalid file. I figured it would be nice to mention this because I believe this will be a frequent cause of the error.
## Changelog
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[General] [Changed] - improve "not a registered callable module error message"
Pull Request resolved: https://github.com/facebook/react-native/pull/28913
Test Plan:
tested locally with RNTester, prints
```
[Sun May 17 2020 18:15:55.396] ERROR Invariant Violation: Module AppRegistry is not a registered callable module (calling runApplication). This can happen when the application entry file path is incorrect.
On Android, verify 'getJSMainModuleName()' in 'MainApplication.java' and 'project.ext.react.entryFile' in 'android/app/build.gradle'.
On iOS, verify '- (NSURL *)sourceURLForBridge:(RCTBridge *)bridge' in 'AppDelegate.m' and 'ENTRY_FILE' env. variable passed to 'react-native-xcode.sh' (if any) in 'Build Phases' -> 'Bundle React Native code and images'.
```
in metro logs
Reviewed By: mdvacca
Differential Revision: D23107228
Pulled By: fkgozali
fbshipit-source-id: 0712ed7e593ba96b041578bafdbefcd09a3994b7