Adding new errors to this package:

1. Go to RPCErrorCodes.ts and add the new code and message to the RPCErrorCodes object. The number used must be less than -32001 (it should just be the next number down from the last error code, serially). The message should be unique.
2. Go to errors.ts and add a new named class that extends the RPCError, passing the new error code you just created. For example:

export class TheErrorYouJustCreated extends RPCError {
  constructor({ ...args } = {}) {
    super({ code: '-code you just created', ...args })
  }
}

3. Go to handleErr function, and make a case for your error to be handled.