Handle createHook errors

[jimmyshiau]

When I have no permission to create hook, github server will return error message

{
  message: Not Found,  
  documentation_url: https://developer.github.com/v3/repos/hooks/#create-a-hook
}

Can you throws NotFound error here?

github.dart/lib/src/common/repos_service.dart

Lines 709 to 717 in 086f904

	Future<Hook> createHook(RepositorySlug slug, CreateHook hook) async {
	ArgumentError.checkNotNull(slug);
	ArgumentError.checkNotNull(hook);
	return github.postJSON<Map<String, dynamic>, Hook>(
	'/repos/${slug.fullName}/hooks',
	convert: (i) => Hook.fromJson(i)..repoName = slug.fullName,
	body: jsonEncode(hook),
	);
	}

[github-actions]

Thanks for submitting an issue! @robrbecker will take a look soon!

[axel-op]

What should be added to the postJSON call is an expected status code of 201 Created, so the API would throw if the returned code was different.