[REQ][csharp-netcore] Support custom exceptions

[bgilbert6]

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

My swagger json outlines that any 400 bad requests should return a specific model, but the generator simply turns all requests that aren't a 200 into an ApiException. It still includes the model, but it isn't used anywhere.

Describe the solution you'd like

It'd be nice if the generator created exception classes for expected model outputs on 400 responses such as: {ModelName}Exception and the exception contain the model error details

Describe alternatives you've considered

Customizing the api.mustache. Not sure where to begin there though