Feature request: Blueprint level error handling for ValidationError

[dsdanielko]

Is there a specific reason why ValidationError can only be changed at the app level using validation_error_callback? #83

It would be great to be able to change it using @api.errorhandler(ValidationError) at the api level for people who are moving to flask-openapi3 from another framework to keep backwards compatibility.

If it is just a lack of contributors issue I could have a go at it also.

Thanks for the help!

[luolingchun]

Welcome to submit a PR.
I have a suggestion, I prefer using parameter passing instead of decorators, for example:

1. @api.get('/book', validation_error_callback=xxx)
2. api = APIBlueprint('/book', __name__, validation_error_callback=xxx