Handle nested ProvisionedThroughputExceeded

[bartelink]

As a side effect of #361, ProvisionedThroughputExceededExceptions get nested:-

17:32:13 I Unhandled  {duration=00:00:00.3333882, events=9, stream="Loan-BC-I-00706146"}
System.AggregateException: One or more errors occurred. (One or more errors occurred. (The level of configured provisioned throughput for the table was exceeded. Consider increasing your provisioning level with the UpdateTable API.))
 ---> System.AggregateException: One or more errors occurred. (The level of configured provisioned throughput for the table was exceeded. Consider increasing your provisioning level with the UpdateTable API.)
 ---> Amazon.DynamoDBv2.Model.ProvisionedThroughputExceededException: The level of configured provisioned throughput for the table was exceeded. Consider increasing your provisioning level with the UpdateTable API.
 ---> Amazon.Runtime.Internal.HttpErrorResponseException: Exception of type 'Amazon.Runtime.Internal.HttpErrorResponseException' was thrown.
   at Amazon.Runtime.HttpWebRequestMessage.GetResponseAsync(CancellationToken cancellationToken)

This updates the active pattern to handle the way in manifests now (doubly nested inside an AggregateException)