[REQ] Fail build on illegal schema if schema should be validated

Is your feature request related to a problem? Please describe.

Hi,
I've recently discovered that the OpenAPI generator (we use the maven plugin, but shouldn't matter as far as what I've seen in the code) doesn't fail if an illegal schema is found. In our particular case, it was $ref with other properties. This check is made and logged in the InlineModelResolver Line 269-279. I expected that this kind of error causes a build failure but it doesn't because it simply logs a warning and returns.

Describe the solution you'd like

The build should fail if validateSpec is true. This can be achieved by throwing an exception after logging the error. This would require to pass this flag to the method as parameter. This seems to be quite difficult. The more feasible approach seems to add a new field that stores if the schema should be validated and use the field value in the method. Then one could set this value when instantiating a new InlineModelResolver (DefaultGenerator Line 283).

Describe alternatives you've considered

I don't know if my proposed change is a good solution from an architectural point of view. I'm open to other suggestions and willing to implement the required changes.

Additional context

Is there a specific reason why these kind of errors do not cause the generator to fail?

Start in InlineModelResolver around lines 269-279 to trace how illegal schemas are logged, then inspect DefaultGenerator around line 283 and the validateSpec setting. Determine how validation state reaches the resolver and verify that an illegal schema causes generation to fail when validateSpec is true without changing behavior otherwise.