access token is now optional

* fix some err checks
* github api url can be configured (e.g. point it at an enterprise instance)

[finishes #89638752 #89633744]

Signed-off-by: Chris Brown <cbrown@pivotal.io>

resources.go

@@ -1,10 +1,11 @@
 package resource
 
 type Source struct {
-	AccessToken string `json:"access_token"`
-
 	User       string `json:"user"`
 	Repository string `json:"repository"`
+
+	GitHubAPIURL string `json:"github_api_url"`
+	AccessToken  string `json:"access_token"`
 }
 
 type CheckRequest struct {