Changing generic `except Exception` to `PyJWTError`

[finomayato]

https://github.com/auth0-blog/auth0-python-fastapi-sample/blob/main/application/utils.py#L58

All pyjwt exceptions inherit from the PyJWTError which we can use instead of the generic Exception.

Source of truth: https://github.com/jpadilla/pyjwt/blob/master/jwt/exceptions.py#L1-L6