UX: Improve error handling for common OmniAuth exceptions (#7991)

This displays more useful messages for the most common issues we see:
- CSRF (when the user switches browser)
- Invalid IAT (when the server clock is wrong)
- OAuth::Unauthorized for OAuth1 providers, when the credentials are incorrect

This commit also stops earlier for disabled authenticators. Now we stop at the request phase, rather than the callback phase.

app/controllers/users/omniauth_callbacks_controller.rb

@@ -91,7 +91,8 @@ class Users::OmniauthCallbacksController < ApplicationController
   end
 
   def failure
-    flash[:error] = I18n.t("login.omniauth_error")
+    error_key = params[:message].to_s.gsub(/[^\w-]/, "") || "generic"
+    flash[:error] = I18n.t("login.omniauth_error.#{error_key}", default: I18n.t("login.omniauth_error.generic"))
     render 'failure'
   end