Improve typing for errorhandler decorator.

[pcorpet]

Today the errorhandler decorator does not have consistency between the type of error in its parameter and the parameter of the function it is wrapping although those should be the same.

The following code should be properly typed:

@app.errorhandler(auth.ExpiredTokenException)
def expired_token(error: auth.ExpiredTokenException) -> Tuple[str, int]:
    """Handle the 498 error."""

    return error.description or i18n.flask_translate("Expired token"), 498

the current version enforces it to be typed as

@app.errorhandler(auth.ExpiredTokenException)
def expired_token(untyped_error: Exception) -> Tuple[str, int]:
    """Handle the 498 error."""

    error = typing.cast(auth.ExpiredTokenException, untyped_error)
    return error.description or i18n.flask_translate("Expired token"), 498

[davidism]

In #4487, I relaxed the type for the callable argument to Any.