Lambda subscription - never recovers from error

[alkampfergit]

This is a more general problem for ISubscriptions: when we have a DB Error (es mongodb is down) and the provider calls the OnError method of the ISubscriber, the LambdaSubscription will mark itself as failed. When the db comes online again, everything starts working again, but noone will clear the error of the Subscription.

We should probably distinguish between error in invoking the function of the ISubscription and generic errors that can happens in the data infrastructure.