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>

cmd/in/in.go

@@ -18,7 +18,11 @@ func main() {
 
 	destDir := os.Args[1]
 
-	github := resource.NewGitHubClient(request.Source)
+	github, err := resource.NewGitHubClient(request.Source)
+	if err != nil {
+		resource.Fatal("constructing github client", err)
+	}
+
 	command := resource.NewInCommand(github, os.Stderr)
 	response, err := command.Run(destDir, request)
 	if err != nil {