Implement exceptions

[Damien Thébault]

Functions should raise proper exceptions.

[Damien Thébault (damien-thebault)]

Exceptions should match the ones from the 'socket' module.
Quoting the python documentation [1]:

"exception gaierror
    This exception is raised for address-related errors, for getaddrinfo() and getnameinfo().[...]"
(res_query/res_search should raise exceptions too)

[1] http://www.python.org/doc/lib/module-socket.html