Update error message on unrecognized commands

Summary:
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Kevin: This isn't quite a typo, but it's a change to an error message.

In my experience, frequently when this error message is displayed, the command is unrecognized because I haven't run npm install, I just checked out a repo of someone else's. We could just update this error message a bit, and I think it would improve the developer experience.
Closes https://github.com/facebook/react-native/pull/7859

Differential Revision: D3379234

fbshipit-source-id: 4fb6e5bae20904871c9c4f7504013dc18b1fe707

react-native-cli/index.js

@@ -108,7 +108,7 @@ if (cli) {
   default:
     console.error(
       'Command `%s` unrecognized. ' +
-      'Did you mean to run this inside a react-native project?',
+      'Make sure that you have run `npm install` and that you are inside a react-native project.',
       commands[0]
     );
     process.exit(1);