Usability improvements for AtPlus

Summary: After working with Jeff, I got AtPlus to work, but it was tricky for me, and would probably be tricky for newbs like me.

My suggestions mainly revolve around this code:
{{ config.BucketConfigs = new Dictionary<string, BucketConfiguration> {
{
"travel-sample", new BucketConfiguration

{ UseEnhancedDurability = true }

}
};}}

Suggestions:
1. If you specify a bucket name that doesn't exist in BucketConfigs, no exception is thrown. If I typo the bucket name, I will receive errors downstream when using AtPlus, and it's not clear that they are related.

2. If I omit the BucketConfigs code, and I later try to use AtPlus, I get an exception "Document does not contain valid MutationToken". It would be nice if the message had additional information like "Make sure to set UseEnhancedDurability to true" or something along those lines.

3. Setting UseEnhancedDurability to true within a dictionary of buckets on config is not very discoverable. I would suggest perhaps a convenience method on ClientConfiguration itself. Maybe SetUseEnhancedDurability(string bucketName, bool value). Alternatively, make UseEnhancedDurabiltiy default to true?