Use errors schema in generated doc. Closes #16

hapic/doc.py

 
     if description.errors:
         for error in description.errors:
+            schema_class = type(error.wrapper.schema)
             method_operations.setdefault('responses', {})\
                 [int(error.wrapper.http_code)] = {
                     'description': str(error.wrapper.http_code),
+                    'schema': {
+                        '$ref': '#/definitions/{}'.format(schema_class.__name__)  # nopep8
+                    }
                 }
 
     # jsonschema based
                     description.output_body.wrapper.processor.schema
                 ))
 
+            if description.errors:
+                for error in description.errors:
+                    schemas.append(type(error.wrapper.schema))
+
         for schema in set(schemas):
             spec.definition(schema.__name__, schema=schema)