Improve error message when cnb buildback fail to download (#4091)

* Improve error message when cnb buildback fail to download

* Reworked error message after input from UX

Author: c0d1ngm0nk3y

errors/v2.yml

@@ -1372,7 +1372,7 @@
 430002:
   name: CNBDownloadBuildpackFailed 
   http_code: 400
-  message: "cnb: downloading buildpacks failed"
+  message: "cnb: downloading buildpacks failed: This may be caused by a wrong url, invalid buildpack or invalid credentials. Check the staging logs for more details."
 
 430003:
   name: CNBDetectFailed