Remove SimpleFIN sync errors modal and related routes, methods, and logic. (#365)

- Removed the `errors` modal and its associated view.
- Eliminated references to `errors` route and controller methods.
- Consolidated error handling into the `register_error` method to improve error tracking and de-duplication.
- Enhanced logging and introduced instrumentation for better observability.

Co-authored-by: Josh Waldrep <joshua.waldrep5+github@gmail.com>

config/routes.rb

@@ -303,7 +303,6 @@ Rails.application.routes.draw do
     member do
       post :sync
       post :balances
-      get :errors
       get :setup_accounts
       post :complete_account_setup
     end