Sparse fieldsets for invalid type results in 500

[ddobson]

Description
When attempting to use sparse fieldsets for an invalid type, a 500 is raised. Is this the correct functionality? I would think this is more of a 400 level error. To demonstrate, if one were to make a request for GET /cars?fields[dealership]=name using the code below, they would get the documented 500 response. I would be happy to open a PR to change this functionality if there is agreement that there is a better way to handle these errors (i.e. raise 4XX error).

Problematic Code

flask-combo-jsonapi/flask_combo_jsonapi/schema.py

Line 160 in 33aede1

raise Exception("Couldn't find schema for type: {}".format(resource_type))

Example

# Example
class Car(Base):
    id = Column(Int, primary_key=True)
    make = Column(Str)
    model = Column(Str)

class CarSchema
    class Meta:
        type_ = "car"

    make = fields.Str()
    model = fields.Str()

class CarListResource(ResourceList):
    methods = ["GET"]
    schema = CarSchema
    data_layer = {"session": db.session, "model": Car}

app = Flask(__name__)
api = Api(app)
api.route(
    CarListResource, "car_resource", "/cars",
)

Example Response

// GET /cars?fields[dealership]=name`
// status code: 500

{
  "errors": [
    {
      "status": "500",
      "title": "Unknown error",
      "detail": "Couldn't find schema for type: dealership"
    }
  ],
  "jsonapi": {
    "version": "1.0"
  }
}