Fix exception unwrapping for unary call

src/csharp/Grpc.Core/Calls.cs

src/csharp/Grpc.Core/Grpc.Core.csproj

src/csharp/Grpc.Core/Internal/AsyncCall.cs

src/csharp/Grpc.Core/RpcException.cs

src/csharp/Grpc.Core/ServerCallHandler.cs

src/csharp/Grpc.Core/Utils/ExceptionHelper.cs