c-ares: improve error message on failed resolve

When the c-ares based resolver backend failed to resolve a name, it
tried to show the name that failed from existing structs. This caused
the wrong output and shown hostname when for example --interface
[hostname] was used and that name resolving failed.

Now we use the hostname used in the actual resolve attempt in the error
message as well.

Bug: http://curl.haxx.se/bug/view.cgi?id=1191
Reported-by: Kim Vandry