Suggest git apply --reject for failed upgrades

Summary:
The upgrade script uses `git apply --3way`:

https://github.com/facebook/react-native/blob/4906f8d28cdbaf1b432494b473a4c61c1e193881/react-native-git-upgrade/cliEntry.js#L368-L369

If you've already upgraded something that React Native upgraded, this will silently fail. For example if you have already upgraded Gradle to the version available in the new version of React Native, the 3-way patch will fail. In this case, the upgraded version is installed into `node_modules` but `package.json` is not updated. More context, discussion, and information is in https://github.com/facebook/react-native/issues/12112#issuecomment-292619346.

Until a more robust solution is implemented, this PR proposes to mitigate the effects of the problem by informing the user of the possible issue and instructing them of a workaround.

In an affected project, I ran `react-native-git-upgrade` after applying my patch.

As expected, I now receive the following output:

![image](https://user-images.githubusercontent.com/789577/38165770-6209ac68-34de-11e8-9d96-7c782e9ef7ce.png)

After running the suggested command, `git status` now shows me the two rejections:

![image](https://user-images.githubusercontent.com/789577/38165796-d10ca7c8-34de-11e8-9037-8427513e3a84.png)

And I can now [manually apply](https://stackoverflow.com/a/28569128/1445366) these changes.

[CLI] [ENHANCEMENT] [react-native-git-upgrade/cliEntry.js] - Provide instructions for upgrading React Native when 3-way merges don't apply
Closes https://github.com/facebook/react-native/pull/18636

Differential Revision: D7603073

Pulled By: hramos

fbshipit-source-id: 32d387e1ee67d8b182d248c585cd33ba94808357