Namespace's default_error_handler is not used anywhere

[singular-value]

https://github.com/python-restx/flask-restx/blob/41b3b591b4be89d5d27e571dd3a75f849d4455ca/flask_restx/namespace.py#L61

Setting a default_error_handler on a Namespace does not do anything. Grepping the codebase shows that a default_error_handler attribute is never used (although can get set). By contrast, Api's _default_error_handler (with underscore at front) is used in the codebase.