Catch missing datasource earlier

[matthewmueller]

Bug description

Right now you can generate a Prisma Client without a datasource. You'll find out about this at runtime:

PrismaClientInitializationError2: Error in data model: Error validating: You defined no datasource. You must define exactly one datasource.

How to reproduce

generator js {
  provider = "prisma-client-js"
}

model User {
  id Int @id
}

Expected behavior

I would have expected to see this error when you run prisma generate

Environment & setup

• OS:
• Database:
• Node.js version:
• Prisma version:

@prisma/cli          : 2.10.1
@prisma/client       : 2.10.1
Current platform     : darwin
Query Engine         : query-engine 7d0087eadc7265e12d4b8d8c3516b02c4c965111 (at node_modules/@prisma/engines/query-engine-darwin)
Migration Engine     : migration-engine-cli 7d0087eadc7265e12d4b8d8c3516b02c4c965111 (at node_modules/@prisma/engines/migration-engine-darwin)
Introspection Engine : introspection-core 7d0087eadc7265e12d4b8d8c3516b02c4c965111 (at node_modules/@prisma/engines/introspection-engine-darwin)
Format Binary        : prisma-fmt 7d0087eadc7265e12d4b8d8c3516b02c4c965111 (at node_modules/@prisma/engines/prisma-fmt-darwin)
Studio               : 0.304.0