Add a default "success" response message, the status code is 200 or 204
depending on whether the method returns any data.
Append any error messages as specified in the `errors` property
of method's remoting metadata.
Move the description of operation's return type to the "success"
response message.
Include error message models in the API models.
Refactor the way how loopback types are converted to swagger data types.
- `modelHelper.LDLPropToSwaggerDataType` is responsible for
producing a valid Swagger Data Type object from LDL object
(be it a property, accepts item or returns item).
- LDLPropToSwaggerDataType picks only fields that are part
of the swagger spec, everything else is excluded from the result.
It's up to the caller to add extra fields like `description`.
- refactor `routeHelper.extendWithType` to accept an additional arg:
the original LDL object. This way it's possible to copy all
type-specific fields to the output object and don't add anything
else.