Add support for Custom Error Codes

[Palani C]

Motivation

In Java implementation, custom application errors can be thrown using RSocketErrorException.
https://github.com/rsocket/rsocket-java/blob/37fc68c68f4b61d826084330a7b0476a456b63da/rsocket-core/src/main/java/io/rsocket/RSocketErrorException.java#L27

But in Js implementation, there is no way to throw custom Errors, All Js Errors are captured and serialized with the same error code APPLICATION_ERROR.

rsocket-js/packages/rsocket-core/src/RSocketMachine.js

Lines 869 to 875 in 23da9b0

 this._connection.sendOne({ 
   code: ERROR_CODES.APPLICATION_ERROR, 
   flags: 0, 
   message: errorMessage, 
   streamId, 
   type: FRAME_TYPES.ERROR, 
 }); 

Desired solution

Add custom Error class RSocketError and use its error code in the error frame.