Prevent break from lack of authentication

[devdrops]

At this moment, this tool can be used for pull requests on both private and public repositories.

• For public ones: diffcs <source>/<project> <pull request id>
• For privates: diffcs <source>/<project> <pull request id> --github-user=<your github user>

But if you forget to add the --github-user=<your github user> flag for private repos, this results in an error with the following output:

This error happens due that the knplabs/github-api library throws an exception when the authentication is required and missing in the access request.

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The idea is to prevent that error: once the given <project> is detected as private, and the --github-user=<your github user> is not provided in the request, to display a message to the user, to inform that the flag is missing and it's required.

[marcelsud]

I think that the Github API returns 404 when a repository is private due to security issues. Maybe we can show a better message like "Repository not found. Maybe it is a private repository and needs authentication"

[devdrops]

Sure! It actually throws an Exception from Guzzle with the given message Not Found. So, in order to submit a change for this issue, we need to update the catch here, checking the exact exception throwed by Guzzle and then apply a better message.