Subscriptions get stored in db even if CreateGitHubWebhook throws an error

[zchelseal]

Issue Description

When user creates a new subscription, it gets added to the subscriptions table in database even if CreateGitHubWebhook throws an error.

Errors Encountered:

• github.GithubException.UnknownObjectException: 404 {'message': 'Not Found', 'documentation_url': 'https://developer.github.com/v3/repos/hooks/#create-a-hook'}
• github.GithubException.BadCredentialsException: 401 {'message': 'Bad credentials', 'documentation_url': 'https://developer.github.com/v3'}