Better error code handling

[Rigner]

https://github.com/PSNRigner/discord-rpc-java/blob/master/src/main/java/net/rigner/discordrpc/ErrorCode.java
https://github.com/PSNRigner/discord-rpc-java/blob/master/src/main/java/net/rigner/discordrpc/impl/RpcConnection.java#L193
https://github.com/PSNRigner/discord-rpc-java/blob/master/src/main/java/net/rigner/discordrpc/DiscordRpc.java#L328

Implement more error codes, with the use of an id instead of using the values array.

[Notoh]

I'll work on this soon, whenever I get time.

[SirYwell]

It looks like this issue is outdated atm. Is there any ongoing work @Notoh?
And, due to the fact, that SUCCESS is not really an error code, I would recommend to change the name to ResponseCode.

[Notoh]

No work currently being done considering I've been extremely busy with exams and other projects, sorry about that.

[techied]

@PSNRigner close or modify?