[ErrorMessage] Change error message when app is not registered

Summary:
So when I first started porting JS files over from LearnGitBranching into a react native project, I some had require errors (for whatever reason) and I hit this error message a decent amount. I eventually understood it had nothing to do with failing to register the component (which btw sounds like some sign-up process, not actually an internal concept) but I figured we could expand on this message and describe why it might be happening.

I'm not 100% sure on what the second half should be, but open to feedback on this
Closes https://github.com/facebook/react-native/pull/826
Github Author: Peter Cottle <pcottle@fb.com>

Test Plan: Imported from GitHub, without a `Test Plan:` line.

Libraries/AppRegistry/AppRegistry.js

@@ -74,7 +74,9 @@ var AppRegistry = {
     );
     invariant(
       runnables[appKey] && runnables[appKey].run,
-      'Application ' + appKey + ' has not been registered.'
+      'Application ' + appKey + ' has not been registered. This ' +
+      'is either due to a require() error during initialization ' +
+      'or failure to call AppRegistry.registerComponent.'
     );
     runnables[appKey].run(appParameters);
   },