Display error when we cannot find an OAuth Application for the Geo node

app/controllers/oauth/geo_auth_controller.rb

 class Oauth::GeoAuthController < ActionController::Base
   rescue_from Gitlab::Geo::RemoteNode::InvalidCredentialsError, with: :invalid_credentials
+  rescue_from Gitlab::Geo::OauthApplicationUndefinedError, with: :undefined_oauth_application
   rescue_from OAuth2::Error, with: :auth
 
   def auth
@@ -39,4 +40,8 @@ class Oauth::GeoAuthController < ActionController::Base
     render :error, layout: 'errors'
   end
 
+  def undefined_oauth_application
+    @error = 'There is no OAuth application defined for this Geo node.'
+    render :error, layout: 'errors'
+  end
 end

lib/gitlab/geo.rb

 module Gitlab
   module Geo
+    class OauthApplicationUndefinedError < StandardError; end
+
     def self.current_node
       RequestStore.store[:geo_node_current] ||= begin
         GeoNode.find_by(host: Gitlab.config.gitlab.host,
@@ -43,7 +45,8 @@ module Gitlab
     def self.oauth_authentication
       return false unless Gitlab::Geo.secondary?
 
-      RequestStore.store[:geo_oauth_application] ||= Gitlab::Geo.current_node.oauth_application
+      RequestStore.store[:geo_oauth_application] ||= Gitlab::Geo.current_node.oauth_application or
+                                                     raise GeoOauthApplicationUndefinedError
     end
   end
 end