How to use the fluture.reject function in fluture

const middleware = (ctx, next) => {
    if (!isValid(ctx.request.body)) {
      // eslint-disable-next-line no-console
      console.error(ctx.request.body, isValid.errors)
      return Future.reject(new ValidationError(isValid.errors))
    }
    return next()
  }

function dhSwitch(uid: string, url: string, auth, response, keys) {
  switch (response._) {
    case 'dh_gen_ok': return /*::joinChain(*/dhGenOk(response, keys, auth)/*::)*/
    case 'dh_gen_retry': return /*::joinChain(*/dhGenRetry(uid, url, response, keys, auth)/*::)*/
    case 'dh_gen_fail': return /*::joinChain(*/dhGenFail(response, keys, auth)/*::)*/
    default: return reject(new Error('Unknown case'))/*::.mapRej(dhAnswerFail)*/
  }
}

function refineResPQ(nonce, response) {
  if (
    typeof response !== 'object'
    || response == null
    || response instanceof Uint8Array
    || Array.isArray(response)
  ) return reject(ERR.sendPQ.noResponse())
  if (response._ !== 'resPQ') {
    return reject(ERR.sendPQ.response(response._))
  }
  const resp: ResPQ = (response: any)
  if (!bytesCmp(nonce, resp.nonce)) {
    return reject(ERR.sendPQ.nonce())
  }
  return of(resp)
}

const assertDhResponse = ({ nonce, serverNonce, newNonce }) => (
  response: Server_DH_Params
)/*:: : Fluture<*, *> */ => {
  if (response._ === 'server_DH_params_fail') {
    const newNonceHash = sha1BytesSync(newNonce).slice(-16)
    if (!bytesCmp(newNonceHash, response.new_nonce_hash)) {
      return reject(ERR.sendDH.hash())/*::.mapRej(assertFail)*/
    }
    return reject(ERR.sendDH.fail())/*::.mapRej(assertFail)*/
  }
  if (response._ !== 'server_DH_params_ok') {
    return reject(ERR.sendDH.invalid(response._))/*::.mapRej(assertFail)*/
  }

  if (!bytesCmp(nonce, response.nonce)) {
    return reject(ERR.sendDH.nonce())/*::.mapRej(assertFail)*/
  }

  if (!bytesCmp(serverNonce, response.server_nonce)) {
    return reject(ERR.sendDH.serverNonce())/*::.mapRej(assertFail)*/
  }
  return of(response)/*::.mapRej(assertFail)*/
}

const assertDhParams = (nonce, serverNonce) => (
  response: Set_client_DH_params_answer
)/*::: Fluture */=> {
  if (checkDhGen(response._)) {
    return reject(ERR.dh.responseInvalid(response._))/*::.mapRej(assertFail)*/
  } else if (!bytesCmp(nonce, response.nonce)) {
    return reject(ERR.dh.nonce.mismatch())/*::.mapRej(assertFail)*/
  } else if (!bytesCmp(serverNonce, response.server_nonce)) {
    return reject(ERR.dh.nonce.server())/*::.mapRej(assertFail)*/
  }
  return of(response)/*::.mapRej(assertFail)*/
}

sub_(dhDec, one)
  const case1 = !greater(gALeemon, one)
  const case2 = !greater(dhDec, gALeemon)
  if (case1)
    return reject(ERR.verify.case1())/*::.mapRej(verifyFail)*/

  if (case2)
    return reject(ERR.verify.case2())/*::.mapRej(verifyFail)*/
  const case3 = !!greater(leemonTwoPow, gALeemon)
  const dhSubPow = dup(dhPrimeLeemon)
  sub(dhSubPow, leemonTwoPow)
  const case4 = !greater(dhSubPow, gALeemon)
  if (case3)
    return reject(ERR.verify.case3())/*::.mapRej(verifyFail)*/
  if (case4)
    return reject(ERR.verify.case4())/*::.mapRej(verifyFail)*/
  innerLog('2^{2048-64} < gA < dhPrime-2^{2048-64} OK')

  const offset = deserializer.getOffset()
  if (!bytesCmp(hash, sha1BytesSync(answerWithPadding.slice(0, offset))))
    return reject(ERR.decrypt.sha1())/*::.mapRej(decryptFail)*/

  return of(response)/*::.mapRej(verifyFail)*/
}

export function safeImport(
    fileName: string
): Future.FutureInstance {
    let moduleObj = null

    try {
        moduleObj = require(fileName)
    } catch (e) {
        return e.code === 'MODULE_NOT_FOUND' ? Future.of(fileName) : Future.reject(e)
    }

    return Future.of(moduleObj)
}

export const userRegister = (registerData) => {
  const usersWithEmail = models.Users.chain().find({
    email: registerData.email,
  })

  if (usersWithEmail.count() > 0) {
    return Future.reject("email_already_exists")
  }

  const createdUser = models.Users.insert({
    email: registerData.email,
    password: registerData.password,
  })

  return Future.of({ userId: createdUser.$loki })
}