Raise Mastodon::RaceConditionError if Redis lock failed (#7511)

An explicit error allows user agents to know the error and Sidekiq to
retry.

app/services/resolve_account_service.rb

@@ -49,6 +49,8 @@ class ResolveAccountService < BaseService
         else
           handle_ostatus
         end
+      else
+        raise Mastodon::RaceConditionError
       end
     end