C# - Add more error information

[Nixarn]

When Nakama throws an exception it would be useful to get more information about the exception. At the moment the only way you can do it is by doing a string comparison on the exception message.

For example when you join a match with an invalid match id, the nakama server return error code 3 (bad input) but the .net client library does not pass on this information. The error code you get in this case is 60 (but you do get the error message "Invalid match ID") and there's no way to look up what that code means.