Add hostname lookup failure to xmlrpc. Fixes #364

Now we get  nice error message if we cannot resolve the hostname of the hostname of the remote machine:

```
$ rostopic info /chatter
Type: std_msgs/String

Publishers: 
 * /talker (http://foo:58605/)

Subscribers: None

$ rosrun roscpp_tutorials listener
Couldn't find an AF_INET address for [foo]
```