When the shop blocked access for an API key due subscriber limit it used
to set the error as "Insufficient privileges," and that was in the plugin
interpreted same as "Subscribers limit reached".
The SHOP-1228 changes the error message will be set to "Subscribers limit reached".
This commit updates the error mapper to be able to process the new error message.
It also keeps the old approach for previously blocked keys.
[MAILPOET-5191]
