Improve error message: Exposing a generated sql type

[matt2e]

If you expose a sql type as a verb request or response type in go you get this error message:
Build failed: could not extract schema: "CreateRequestQuery" is an unsupported generated type: *schema.Data
We should make it clearer, explaining that sql types must be private to the module.

[matt2e]

It would also be interesting if we could move this check to schema validation so it works for all languages