Improve error when using prisma migrate --experimental with native types

[matthewmueller]

Bug description

Right now we throw an unexpected error when you try to save a migration with nativeTypes.

How to reproduce

// This is your Prisma schema file,
// learn more about it in the docs: https://pris.ly/d/prisma-schema

datasource db {
  provider = "postgresql"
  url      = env("DATABASE_URL")
}

generator client {
  provider        = "prisma-client-js"
  previewFeatures = ["nativeTypes"]
}

model User {
  id    Int    @id @default(autoincrement())
  first String
  last  String
  posts Post[]
}

model Post {
  id       Int    @id @default(autoincrement())
  title    String
  body     String
  user     User   @relation(fields: [userId], references: [id])
  userId   Int
  visitors BigInt
}

npx prisma migrate save --experimental
Environment variables loaded from /Users/m/Go/src/github.com/prisma/qa/.env
Prisma schema loaded from prisma/schema.prisma
Response "Some of the requested preview features are not yet allowed in migration engine. Please remove them from your data model before using migrations. (blocked: `nativeTypes`)"
Got result for unknown id undefined
Oops, an unexpected error occured!
Error in migration engine: Nov 09 10:42:13.778  INFO migration_engine: Starting migration engine RPC server git_hash="a624a6628b9e5e453bb8f8cb0460e233a3a97e62"

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Expected behavior

An error, but an expected one. We'll get lots of error reports otherwise.

Environment & setup

Environment variables loaded from /Users/m/Go/src/github.com/prisma/qa/.env
@prisma/cli          : 2.11.0-dev.43
@prisma/client       : 2.11.0-dev.43
Current platform     : darwin
Query Engine         : query-engine a624a6628b9e5e453bb8f8cb0460e233a3a97e62 (at node_modules/@prisma/engines/query-engine-darwin)
Migration Engine     : migration-engine-cli a624a6628b9e5e453bb8f8cb0460e233a3a97e62 (at node_modules/@prisma/engines/migration-engine-darwin)
Introspection Engine : introspection-core a624a6628b9e5e453bb8f8cb0460e233a3a97e62 (at node_modules/@prisma/engines/introspection-engine-darwin)
Format Binary        : prisma-fmt a624a6628b9e5e453bb8f8cb0460e233a3a97e62 (at node_modules/@prisma/engines/prisma-fmt-darwin)
Studio               : 0.308.0
Preview Features     : nativeTypes