http: improve error path for bufferevent_{setfd,enable,disable}()

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

(cherry picked from commit b98d32d0c90864ff60bc689f3bcf738af58ca79c)