use async create exception in async methods (#22952)

* update name from tryget to getifexists

* update to return Response<T> for GetIfExists and CheckIfExists

* update api

* update ManageGroup to adop if exists changes

* update subscription to adopt if exists changes

* update features to use new if exists

* update generic resource to use new if exists

* update provider to use new if exists

* update resource group to use new if exists

* final changes

* update api after changes

* update snippets to not use var
update typo in readme

* one more typo

* update list references

* update to use async create exception

Author: m-nash