Add human-readable error for paywalled series

[hurlenko]

From https://github.com/hurlenko/mloader/issues/36#issuecomment-1754870773:

As you probably know Shueisha locked some series behind a paywall of subscription. These series only have 3 first chapters available and after downloading them, the script throws an error. Assassination Clasroom (100001) is an example. Would be great if the error could throw some human readable message.