[LLC] Remove exception allocation from generated "Exists" method

[annelo-msft]

To resolve this issue, we're pre-allocating an exception in the default: block of this switch statement.

Once this issue has been completed, we will have all the pieces we need in place to wait to allocate the exception until response.Value is called, i.e. we believe we will have a public type called OptionalResponse<T>, and in its implementation of the Value property, it can use the new constructor on RequestFailedException to create the exception.

Depends on completion of: Azure/azure-sdk-for-net#23372

[annelo-msft]

Update: OptionalResponse<T> was implemented as ErrorResponse<T>

Per discussion regarding calling the sync constructor where we're currently calling an async method:

[Yesterday 10:12 AM] @pakrym
Most of the time the response is buffered and it's OK to use the sync version. There are some requests that we don't buffer and for them calling CreateRequestFailedException will result in a sync read from a network connection.

Another potential issue here is that HttpMessage is disposed at the end of the service method call, and so does the response. So you need to check if it is possible to extract exception using CreateRequestFailedException after HttpMessage is disposed.

Same problem here. For buffered responses content will still be available. For non-buffered content will be a disposed network stream.
[Yesterday 10:12 AM] @pakrym
I wonder if we need to add Response.BufferContentAsync() or just a feature that always buffers error responses as part of the pipeline
It will make the error handling story way simpler if we can presume that all failed responses have a buffered content.

Let's add the BufferContent() and BufferContentAsync() to Response, but wait to do this performance optimization.

[chunyu3]

Hi @annelo-msft What's purpose of this issue? Is it to replace the current exception-generate method (clientDiagnostics.CreateRequestFailedException) to use RequestFailedException method. see following:
Current:

update to:

[annelo-msft]

Since this method is never used, let's not block GA on it.

[chunyu3]

@annelo-msft Is the issue to replace the exception-generate method as above comment? Thanks