Throw meaningful error on non-supported providers

[Maximvdw]

Currently when 'trying' to get the notification gateway from a pod provider; it will extract the gateway
from .well-known/solid. Not supported providers will not have the notificationGateway option.
This causes a fetch on an undefined URL causing an non-descriptive error:

solid-client-notifications-js/src/notification.ts

Line 166 in 4eda50a

const response = await this.fetch(this.gateway, {

Would it be an option to throw a more meaningful error before the above line that identifies the provider as unsupported?