Redis.CheckNameAvailability pattern is not implemented correctly

Current latest spec describes return type of an operation `checkNameAvailability` as void.
https://github.com/Azure/azure-rest-api-specs/blob/master/specification/redis/resource-manager/Microsoft.Cache/stable/2018-03-01/redis.json#L91

In the generated code the method will be as `void checkNameAvailability(parameters)` and if it will not throw an exception (C#/Java) then the name is available.

Start with specification/redis/resource-manager/Microsoft.Cache/stable/2018-03-01/redis.json at the checkNameAvailability operation and review its declared response. Compare that definition with the generated C# and Java method behavior described in the issue; done means the specification and generated operation follow the intended name-availability pattern.

Written by the indexing model from the issue text.