Fix the parse error when specifying an unknown version.

Summary:
**Motivation**

When running `react-native-git-upgrade` with an unknown version, the error message isn't very helpful

**Test Plan**

- Publish the `master` branch to Sinopia
- Run `react-native-git-upgrade 0.666.0` inside a RN project
- Error message is `SyntaxError: Unexpected end of JSON input`
- Publish this branch to Sinopia
- Run `react-native-git-upgrade 0.666.0` inside a RN project
- Error message should be `Error: The specified version of React Native 0.666.0 doesn't exist.
Re-run the react-native-git-upgrade command with an existing version,
for example: "react-native-git-upgrade 0.38.0",
or without arguments to upgrade to the latest: "react-native-git-upgrade".`
Closes https://github.com/facebook/react-native/pull/11264

Differential Revision: D4265553

Pulled By: mkonicek

fbshipit-source-id: 8597eb09cc3397bfa6d2205a9b3bb30acfad530f