Drop support for flask 0.x and register default error handler only for HTTPException

[lafrech]

Since Flask 1.0, it is now possible to register a error handler for generic HTTPException (pallets/flask#941, pallets/flask#2314).

Because this is not possible with Flask 0.x, the error handler is registered for all error codes as a workaround, which can mess with other extensions registering error handlers if flask-rest-api is initiated last.

This PR addresses this by only registering the error handler for HTTPException and dropping support for Flask 0.x. Now other extensions may register error handlers for specific exceptions or codes.

[coveralls]

Coverage decreased (-0.0007%) to 99.625% when pulling 5d22e5c on dev_drop_flask_0.x into d42641a on master.