Bug 241272

Currently, attempting `git-webkit setup` with an access token that has expired causes the setup script to offer to create a WebKit fork; picking either option (Yes or No) causes the script to fail.

```
% gw setup
WARNING:webkitscmpy:Setting merging behavior for changelogs...
WARNING:webkitscmpy:Set merging behavior for changelogs!
Create a private fork of 'WebKit' belonging to 'whsieh' ([Yes]/No): Yes
Failed to create a fork of 'WebKit' belonging to 'whsieh'
URL: https://api.github.com/repos/WebKit/WebKit/forks
Server replied with status code 401:
{"message":"Bad credentials","documentation_url":"https://docs.github.com/rest"}
Setup failed
% gw setup
WARNING:webkitscmpy:Setting merging behavior for changelogs...
WARNING:webkitscmpy:Set merging behavior for changelogs!
Create a private fork of 'WebKit' belonging to 'whsieh' ([Yes]/No): No
Setup failed
```

The workaround was to regenerate my expired token via https://github.com/settings/tokens and update the token stored in my keychain; it would be nice if the script detected this scenario and surfaced a better error message.