Add support for Custom Error Codes

[palamccc]

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.