Inform customers with reseller subscriptions why they are not able to login

Problem

Customers who purchased their subscription through a reseller should not have access to the Customers Portal.
An MVC was implemented in #1373 (closed).

Administrators can see a banner explaining that the customer is blocked.
However, the customer themselves see nothing.
Their logins fail, and password reset emails never arrive (because they're not sent).

As these customers are completely left in the dark why their login fails, they create support tickets.

Proposal

When a user with a reseller subscription tries to login or request a password reset, show a message explaining why they are not allowed to login.