http: improve error path for bufferevent_{setfd,enable,disable}() We have calls to the next functions but do not check return values, though they can be invalid and it is better to show this somehow. Also do bufferevent_setfd() first and only after it bufferevent_enable()/bufferevent_disable() since: a) it is more natural b) it will avoid extra operations c) it will not fail first bufferevent_enable() (this is the case for buffbufferevent_async at least) In this case we could add more information for issues like #709