Extract code that runs on error and success finish conditions when importing subscriptions to separate private functions.

Better maintainability this way.
When enqueuing a job to subscribe to a feed, adapt to changes in arguments to SubscribeUserWorker in 787ff762b295f2299f5458af4f7e9e9c9022d2d7

SubscribeUserWorker now is only enqueued when the user requests a new subscription. It is never part of an OPML import, and the arguments related to OPML import have been removed.
Remove folder_id argument from SubscribeUserWorker#perform

It is not necessary to pass the folder id anymore. Since recent changes, SubscribeUserWorker is enqueued only when the user manually requests a new subscription; this means that a nil was always being passed in this argument. Better to get rid of it.

This makes a private method so simple that it's been removed and its code incorporated to the main perform method.
Renamed private function.
Worker validation was missing a return statement if the validation failed.
WIP: adapt tests to recent changes in subscribe and OPML import workers.