Add specific GRPC errors mock

[eudelins-zama]

Currently, then error method of the Then struct takes an HTTP StatusCode (and a message).

But I did not find any way to return custom GRPC error using GRPC Code instead. So currently, I don't think there is a way to mock AlreadyExists or ResourceExhausted GRPC errors for example.

It would be nice to add a grpc_error method to the Then struct or whatever to support this.

[declark1]

We actually had something like this initially, but wanted to standardize around regular HTTP status codes in the mock response type since mocktail is hybrid HTTP + gRPC and most common gRPC codes have an equivalent (or close enough) HTTP code. I can definitely see this being useful for those codes that don't have a 1:1 mapping, so will look into this to see if we can cleanly support both.