throw an exception on id.get if session is not available

[iamkrillin]

Throw exception on id.get

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Summary of the changes (Less than 80 chars)

Description

Throw an exception in DistributedSession.Id.Get if the session is not available

[dotnet-policy-service]

Thanks for your PR, @iamkrillin. Someone from the team will get assigned to your PR shortly and we'll get it reviewed.

[amcasey]

It looks like the unavailability of a session is already signalled by an empty Id. Is that not sufficient?

[iamkrillin]

The problem I was running into was two fold:

1. The documentation says the Id returned is unique, which strictly speaking, if there is a possibility for it to return "" is not always true, but the bigger problem was..
2. When using sqlserverdistributedcache, if there is a problem with the setup (server not available, table missing, etc). The error gets eaten then logged to the console (which is not ideal when using IIS hosting)

These two things combined lead to a scenario where things looked to be working, but were not working. When I was working on this PR I though it was better to throw an exception since that would make it very clear that things were not working, but returning null would also work

Either of these would be preferable to eating a error, then returning a value IMO

[amcasey]

I'm not actually familiar with this type, but the current behavior certainly appears to be intentional. If we were going to move to a model where it throws, wouldn't it make more sense to just add a throw in Loads catch block?

Similarly, it would have been easy to give each failed session its own guid, so I assume they were grouped together with the id "" on purpose. (Whether that purpose is valid, I have no idea, though it's a common enough pattern.)

[iamkrillin]

That's a fair enough point, I did consider adding a throw into the catch in Load, I ultimately decided against it since Load is called in many places and I wanted my change to be as small as possible.