Improve OAuth signup error message.

app/controllers/omniauth_callbacks_controller.rb

@@ -65,8 +65,15 @@ class OmniauthCallbacksController < Devise::OmniauthCallbacksController
         redirect_to omniauth_error_path(oauth['provider'], error: error_message) and return
       end
     end
-  rescue Gitlab::OAuth::ForbiddenAction => e
-    flash[:notice] = e.message
+  rescue Gitlab::OAuth::SignupDisabledError => e
+    message = "Signing in using your #{oauth['provider']} account without a pre-existing GitLab account is not allowed."
+
+    if current_application_settings.signup_enabled?
+      message << " Create a GitLab account first, and then connect it to your #{oauth['provider']} account."
+    end
+
+    flash[:notice] = message
+    
     redirect_to new_user_session_path
   end

lib/gitlab/o_auth/user.rb

@@ -5,7 +5,7 @@
 #
 module Gitlab
   module OAuth
-    class ForbiddenAction < StandardError; end
+    class SignupDisabledError < StandardError; end
     class User
       attr_accessor :auth_hash, :gl_user
@@ -99,7 +99,7 @@ module Gitlab
       end
       def unauthorized_to_create
-        raise ForbiddenAction.new("Unauthorized to create user, signup disabled for #{auth_hash.provider}")
+        raise SignupDisabledError
       end
     end
   end