How and if test syntactic (bad request) validation with Pact?

I'm starting to use Pact (on Java) for Contract Tests.

I've read Contract Tests vs Functional Tests on Pact best practices but I'm a little confused.

Example: a simple REST endpoint that creates a resource (POST), returning a 201 Created on success and 400 Bad Request for syntactic validation errors. The request body is something like:

{
    "firstname" : "Foo",
    "lastname" : "Bar"
}

Both firstname and lastname must not be blank. So far as I understand I could write 3 scenarios here, in which the provider should return 400 Bad Request:

1. firstname is blank, lastname is not blank
2. firstname is not blank, lastname is blank
3. firstname and lastname are both blank

The thing is that the mock server returns a 500 Internal Server Error if only the "happy path pact" is defined, and if I want to make it return 400 Bad Request I have to write all possible pacts. Moreover, if I add another fields with the same validation rule the number of pacts explodes.

• First of all, should I test that scenarios on my consumer?
• If yes, do exists any smart way to implement it with Pact DSL?

Thank you.