Github PR plugin not working: the plugin should warn the user that the server is not configured and point the user to the documentation.

[sunix]

Describe the bug

By default, having a workspace with the Github PR plugin is not working.
It is doing something at start up with a popup. But then ... nothing.
In most of the case, server should have Github oauth activated in the Che server https://www.eclipse.org/che/docs/che-7/end-user-guide/configuring-github-oauth/

Che version

• latest

Expected behavior.

User should see an error saying that it couldn't perform the oauth and point the user to the documentation.

[sunix]

dups #18237